Fictive temperature in damage-resistant glass having improved mechanical characteristics

ABSTRACT

A strengthened glass sheet product as well as process and an apparatus for making the product. The process comprises cooling the glass sheet by non-contact thermal conduction for sufficiently long to fix a surface compression and central tension of the sheet. The process results in thermally strengthened glass sheets having improved breakage properties.

RELATED APPLICATIONS

This application claims the benefit of U.S. Application No. 62/031,856filed Jul. 31, 2014, U.S. Application No. 62/074,838 filed Nov. 4, 2014,and U.S. Application No. 62/147,289 filed Apr. 14, 2015, each of whichis incorporated by reference herein in its entirety.

BACKGROUND

This disclosure relates to improved thermally conditioned (strengthenedor tempered) glass, particularly glass sheets, and improved methods andapparatuses for the thermal strengthening of glass, particularly forglass sheets.

In thermal (or “physical”) strengthening of glass sheets, a glass sheetis heated to an elevated temperature above the glass transitiontemperature of the glass, then the surfaces of the sheet are rapidlycooled (“quenched”), while the inner regions of the sheet, insulated bythe thickness and fairly low thermal conductivity of the glass, cool ata slower rate. This differential cooling produces a residual compressivestress in the glass surface regions, balanced by a residual tensilestress in the central regions of the glass. This is distinguished fromchemical strengthening of glass, in which surface compressive stressesare generated by changing the chemical composition of the glass inregions nearer the surface, relative to the center, such as by iondiffusion. This also is distinguished from glass strengthening bycombining or laminating together, while hot, layers of glasscompositions having differing coefficients of thermal expansion, withlower expansion layers typically outermost, to result in surfacecompressive stresses upon return to ambient temperature. Relative tochemical strengthening and lamination, thermal strengthening processesare generally less expensive and much quicker to perform.

Thermally strengthened glass has advantages relative to unstrengthenedglass. The surface compression of the strengthened glass providesgreater resistance to fracture than unstrengthened glass. The increasein strength generally is proportional to the amount of surfacecompression. If a sheet possesses a sufficient level of thermalstrengthening, relative to its thickness, then when and if the sheet isbroken, it will divide into small fragments with dull edges rather thaninto large or elongated fragments with sharp edges. Glass that breaksinto sufficiently small fragments, or “dices,” as defined by variousestablished standards, may be known as safety glass, or “fully tempered”glass, or sometimes simply “tempered” glass.

Because the degree of strengthening depends on the temperaturedifference between the surface and center of the glass sheet, thinnerglasses require higher cooling rates to achieve a given stress. Also,thinner glass generally requires higher final values of surfacecompressive stress and central tension to achieve dicing into smallparticles upon breaking. Accordingly, achieving full tempering (dicing)in glass with sheet thicknesses of around 3 mm or less has beenexceedingly challenging if not impossible.

SUMMARY

This disclosure relates, in part, to highly strengthened thin glasssheets and methods processes, and apparatuses that achieve surprisinglyhigh levels of heat strengthening of glass sheets at thicknesses notachieved in the past exceeding the current state of the art ofconvective gas thermal strengthening of glass, desirably whilecontacting the glass only with a gas and while also decreasing the powerrequirements of the process. The apparatuses and methods disclosedenable thermal strengthening, including up to “full temper” or dicingbehavior, in glass sheets having thicknesses down to at least as thin as0.1 mm.

According to an embodiment of the present disclosure, a strengthenedglass sheet is provided. The glass sheet has a thickness, expressed inmillimeters, of t, a length, expressed in millimeters, of l, and awidth, expressed in millimeters, of w, t being less than l and less thanw. The glass sheet also has a first major surface and a second majorsurface separated by the thickness t, the first major surface of thesheet being flat to 100 μm total indicator run-out (TIR) along any 50 mmor less profile of the first major surface. The glass sheet includes aglass having a softening temperature, expressed in units of ° C., ofT_(soft) and an annealing temperature, expressed in units of ° C., ofT_(anneal), and a surface fictive temperature measured on the firstmajor surface of the glass sheet represented by Tfs, when expressed inunits of ° C. The glass sheet has a non-dimensional surface fictivetemperature parameter θs given by(Tfs−T_(anneal))/(T_(soft)−T_(anneal)), wherein the parameter θs is inthe range of from 0.20 to 0.9.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (Prior Art) is a graph of blower power required for “fulltempering” as a function of glass thickness.

FIG. 2 (Prior Art) is a graph of blower power required for “fulltempering” as a function of glass thickness for an old process ormachine O and a new process or machine N.

FIG. 3 (Prior Art) is a graph of the old curve O and the new curve N ofFIG. 2 scaled to match and superimposed upon the graph of FIG. 1.

FIG. 4 is a diagrammatic cross partial cross section of a thermallystrengthened glass sheet according to one or more embodiments of thepresent disclosure.

FIG. 5 is a plot of the a non-dimensional surface fictive temperatureparameter θs for fictive temperatures obtained by one or moreembodiments of methods and apparatuses of the present invention.

FIG. 6 is a plot of surface compression stresses calculated bysimulation for differing glass compositions, plotted against a proposedtemperability parameter Ψ for the various compositions shown.

FIGS. 7 and 8 are graphs of two parameters P1 and P2 as a function ofheat transfer coefficient h.

FIG. 9 is a graph of MPa of surface compression of a glass sheet as afunction of thickness t of the sheet in millimeters, showing regions ofperformance newly opened by one or more embodiments of the apparatusesand methods of the present disclosure.

FIG. 10 is a graph showing compressive stress as a function of thicknessplotted for selected examples of tempered glass sheets of the presentdisclosure.

FIG. 11 is a flow chart illustrating some aspects of a method accordingto the present disclosure.

FIG. 12 is a flow chart illustrating some aspects of another methodaccording to the present disclosure.

FIG. 13A is the graph of FIG. 3 with a region R and points A, B, A′ andB′ marked thereon to show a region in which the methods and methods andapparatuses and processes of the present disclosure allow operation, incontrast to the prior art.

FIG. 13B is another representation of the region R and points A, B, A′and B′ of FIG. 13A, but shown adjacent to (and positioned relative tothe scale) of a reduced size copy of FIG. 2.

FIG. 14 (Prior Art) is a graph of the required heat transfer coefficientneeded for tempering as a function of glass thickness.

FIG. 15 is a diagrammatic cross-section of a glass sheet being cooled byconduction more than by convection, according to the present disclosure.

FIG. 16 is a schematic cross-sectional diagram of an experimentalapparatus according to the present disclosure.

FIG. 17A is a perspective cut-away view of another embodiment of anapparatus similar to that of FIG. 16.

FIG. 17B is a perspective cut-away view of an alternative embodiment ofthe inset feature of FIG. 17A

FIG. 17C is a perspective cut-away view of yet another alternativeembodiment of the inset feature of FIG. 17A.

FIG. 18 is a flow chart illustrating some aspects of yet another methodaccording to the present disclosure.

DETAILED DESCRIPTION

There is a need for improvements in thermal processing of glass, both inmethods and apparatuses for thermally strengthening glass and theresulting thermally strengthened sheets themselves. For example, inportable electronics there is a desire for thinner, but strongeroptical-quality glass sheet materials and products comprising such glasssheets. Glass is very strong in compression but relatively weak againsttension at the surface. By providing compression at the surface of asheet, balanced by tension at the center where there is no exposedsurface, the useful strength of a glass sheet is dramatically increased.However, while thermal strengthening is generally cheaper and fasterrelative to alternative methods of strengthening, it has suffered fromlimitations on its ability to be used in strengthening thin—e.g., 2-3 mmor less—glass sheets, because the level of strengthening depends on thetemperature difference between the surface and center of the glass sheetand it is difficult to achieve a significant difference between thesurface and center of a thin glass sheet. The present descriptionprovides improved methods and apparatuses for utilizing thermalstrengthening to produce highly strengthened thin glass sheets. Themethods and apparatuses solve the limitations in current processes,allowing for high levels of strengthening in glass sheets withthicknesses less than about 3 mm, less than 2 mm, less than 1.5 mm, lessthan 1.0 mm, less than 0.5 mm, less than about 0.25 mm, and less thanabout 0.1 mm.

Standard industrial processes for thermally strengthening glass involveheating glass sheets in a radiant energy furnace or a convection furnace(or a “combined mode” furnace using both techniques) to a predeterminedtemperature, then gas cooling (“quenching”), typically in the form oflarge amounts of ambient air blown against or along the glass surface.This gas cooling process is predominantly convective, whereby the heattransfer is by mass motion (collective movement) of the fluid, viadiffusion and advection, as the gas carries heat away from the hot glasssheet.

Certain factors can restrict the amount of strengthening possible inglass sheets. Limitations exist, in part, because the amount ofcompressive stress on the finished sheet is related directly to the sizeof the temperature differential, between the surface and the center ofthe sheet, achieved during quenching. However, the larger thetemperature differential during quenching, the more likely glass is tobreak. Breakage can be reduced, for a given rate of cooling, by startingthe quench from a higher initial temperature of the sheet. Also, higherstarting temperatures are known to be necessary to achieve the fullstrengthening potential of higher cooling rates. But increasing thetemperature of the sheet at the start of the quench can lead toexcessive deformation of the sheet as it becomes softer, again limitingthe practically achievable temperature differential.

Sheet thickness also imposes significant limits on the achievabletemperature differential during quenching. The thinner the sheet, thelower the temperature differential between the surface and the centerfor a given cooling rate during quenching, because there is less glassthickness to thermally insulate the center from the surface.Accordingly, thermal strengthening of thin glass requires higher coolingrates and, thus, faster removal of heat from the external surfaces ofthe glass, requiring significant energy consumption. FIG. 1 shows a thepower in kilowatts per square meter of glass sheet area required by airblowers employed to blow sufficient ambient air to “fully temper” sodalime glass (“SLG”), as a function of glass thickness in millimeters,based on industry standard thermal strengthening processes of about 35years ago. The power required increases exponentially as the glass usedgets thinner, thus glass sheets of about 3 mm in thickness were thethinnest fully tempered commercial glass available for many years.Further, the thinner the sheet, the greater the likelihood ofdeformation at a given softness (that is, at a given viscosity) of theglass. Therefore, decreasing thickness both reduces the achievabletemperature differential directly and, because of increased risk ofdeformation of the sheet, tends to reduce the opportunity to use highersheet temperatures to achieve the full benefits of higher cooling ratesand to prevent glass breakage caused by higher cooling rates.

More recently, the performance curves of FIG. 2 (Prior Art) werepublished using state of the art glass thermal strengthening equipment.This improved equipment continues to use traditional air blownconvective processes to cool the glass, but replaces rollers used tosupport the glass during heating with a system that utilizes air tosupport the glass during at least the last stages of heating. Withoutroller contact, the glass can be heated to higher temperatures (andhigher softness/lower viscosity) prior to quenching, reportedly allowingthe production of fully tempered glass at 2 mm thickness. As shown inFIG. 2, the reported blower power required to strengthen a 2 mm thicksheet is reduced from 1200 kW/m² to 400 kW/m² at the higher temperaturesenabled by using air to support the glass (curve N) as compared to usingrollers (curve O).

Although it represents progress to be able to produce fully tempered 2mm thick glass, scaling the old and new curves O and N of FIG. 2 tomatch the scale of FIG. 1, as shown in FIG. 3 (Prior Art), shows thatthe improvement in performance achieved by the new process is relativelysmall and simply an incremental change in the previous understanding ofthe energy needs in convective strengthening of glass sheets. In FIG. 3the old and new curves O and N of FIG. 2 are scaled to match the graphof FIG. 1, and overlaid thereon (with the old curve O truncated at thetop at 240 kW/m² for easier viewing of the new curve N). From FIG. 3 itis apparent that the technology represented by the curve N changes onlyslightly the performance curve of convective gas quenching processestoward the thin glass side. The high operating point (400 kW/m² ofblower power for 2 mm glass) shows the extreme increase in power stillrequired to process thinner glass by this method. The sharp increase inairflow and, thus, power needed suggests the difficulty, as a matter ofboth engineering practice and economics, in going below 2 mm thicknesswhile producing fully tempered glass using conventional convective gasstrengthening methods. Additionally, the very high airflows needed alsocould deform the shape of thinner sheets. Accordingly, to reach fulltemper of glass having a thickness of less than 2 mm or to reach fulltemper at 2 mm in glasses having coefficients of thermal expansion(“CTE”) lower than that of soda lime glasses, another method is needed.

Alternative methods to current commercial convective gas strengtheninghave been tried as well, but each has certain drawbacks relative toconvective gas strengthening. In particular, methods of achieving highercooling rates generally require at least some liquid or solid contactwith the sheet surfaces, rather than only gas. As described in moredetail below, such contact with the glass sheet can adversely affectglass surface quality, glass flatness, and/or evenness of thestrengthening process. These defects sometimes can be perceived by thehuman eye, particularly when viewed in reflected light.

Liquid contact strengthening, in the form of immersion in liquid bathsor flowing liquids, as well as in the form of spraying, has been used toachieve higher cooling rates than convective gas strengthening, but hasthe drawback of causing excessive thermal variations across a sheetduring the cooling process. In immersion or immersion-like spraying orflowing of liquids, large thermal variations over small areas can occurdue to convection currents that arise spontaneously within the liquidbath or liquid flow. In finer spraying, the discrete spray droplets andthe effects of nozzle spray patterns also produce significant thermalvariations. Excessive thermal variations tend to cause glass breakageduring thermal strengthening by liquid contact, limiting the coolingrates and resulting strengths that can be achieved. The necessaryhandling of the sheet (to position or hold it within the liquid bath orliquid flow or liquid spray) also causes physical stress and excessivethermal variations from physical contact with the sheet, tending also tocause breakage during strengthening and limiting the cooling rates andresulting strengths. Finally, some liquid cooling methods, such as highcooling rate quenching by oil immersion and various spraying techniques,can alter the glass surface during such cooling, requiring later removalof glass material from the sheet surface to produce a satisfactoryfinish.

Solid contact thermal strengthening involves contacting the surface ofthe hot glass with a cooler solid surface. As with liquid contactstrengthening, excessive thermal variations, like those seen in liquidcontact strengthening, can easily arise during the quenching process.Any imperfection in the surface finish of the glass sheet, or in thequenching surfaces, or in the consistency of the thickness of the sheet,results in imperfect contact over some area of the sheet, causing largethermal variations that tend to break the glass during processing, andresulting in unwanted birefringence if the sheet survives. Additionally,contacting the hot glass sheet with a solid object can lead to theformation of surface defects, such as chips, checks, cracks, scratches,and the like. Achieving good contact over the entirety of the surfacesof a sheet also can become increasing difficult as the dimensions of thesheet increase. Physical contact with a solid surface also can stressthe sheet mechanically during quenching, adding to the likelihood ofbreaking the sheet during the process. Further, the extremely high ratetemperature changes at the initiation of contact can cause breakageduring sheet processing and, as such, contact cooling of thin glasssubstrates has not been commercially viable.

The present disclosure surpasses the traditional processes describedabove to effectively, efficiently, and evenly thermally strengthen thinglass sheets at commercial scales without damaging the surface of theglass, inducing birefringence or uneven strengthening, or causingunacceptable breakage. Conventionally in convective gas glassstrengthening, higher rates of cooling are achieved by increasing therate of air flow, decreasing the distance of air nozzle openings to theglass sheet surface, increasing the temperature of the glass (at thestart of cooling), and optionally, decreasing the temperature of thecooling air.

Previously unobtainable glass sheets can be produced by one or more ofthe embodiments disclosed herein. This is a result of providing veryhigh heat transfer rates in a precise manner, with good physical controland gentle handling of the glass. Control of (form and) flatness in asmall-gap gas bearing allows for processing sheets at higher relativetemperatures at the start of cooling resulting in higher thermalstrengthening levels. As described below, the result is glass sheetswith unique properties.

Some embodiments of glass sheets treated by methods and/or apparatusesaccording to the present disclosure have higher levels of permanentthermally induced stresses than previously known. Without wishing to bebound by theory, this is believed that the achieved levels of thermallyinduced stress were obtainable for a combination of reasons. The highuniformity of the heat transfer in the processes detailed herein reducesor removes physical and unwanted thermal stresses in the glass, allowingglass sheets to be tempered at higher heat transfer rates withoutbreaking. Further, the present methods can be performed at lower glasssheet viscosities (higher initial temperatures at the start of quench),while still preserving the desired (form and) flatness, which provides amuch greater change in temperature in the cooling process, thusincreasing the heat strengthening levels achieved.

A first embodiment comprises a thermally strengthened glass sheet havinga high surface compressive stress or a high central tension. FIG. 4 is adiagrammatic cross partial cross section of a thermally strengthenedglass sheet 500 according to one or more embodiments. The glass sheet500 has a thickness t and first and second major surfaces 510, 520separated by the thickness t. Glass sheet 500 also includes a length land a width w. In exemplary embodiments, thickness t of glass sheet 500is less than length l of glass sheet 500. In other exemplaryembodiments, thickness t of glass sheet 500 is less than width w ofglass sheet 500. In yet other exemplary embodiments, thickness t ofglass sheet 500 is less than length l and width w of glass sheet 500.The glass sheet 500 further has regions of permanent thermally inducedcompressive stress 530 and 540 at and/or near the first and second majorsurfaces 510, 520, balanced by a region of permanent thermally inducedcentral tensile stress 550 (i.e., tension) in the central portion of thesheet.

Compressive stresses of glasses resulting from the processes disclosedherein can vary as a function of thickness of the glasses. Inembodiments, glasses having a thickness of 3 mm or less have acompressive stress of at least 80 MPa, at least 100 MPa, at least 150MPa, at least 200 MPa, at least 250 MPa, at least 300 MPa, at least 350MPa, at least 400 MPa, and/or no more than 1 GPa. In contemplatedembodiments, glasses having a thickness of 2 mm or less have acompressive stress of at least 80 MPa, at least 100 MPa, at least 150MPa, at least 175 MPa, at least 200 MPa, at least 250 MPa, at least 300MPa, at least 350 MPa, at least 400 MPa, and/or no more than 1 GPa. Incontemplated embodiments, glasses having a thickness of 1.5 mm or lesshave a compressive stress of at least 80 MPa, at least 100 MPa, at least150 MPa, at least 175 MPa, at least 200 MPa, at least 250 MPa, at least300 MPa, at least 350 MPa, and/or no more than 1 GPa. In contemplatedembodiments, glasses having a thickness of 1 mm or less have acompressive stress of at least of at least 80 MPa, at least 100 MPa, atleast 150 MPa, at least 175 MPa, at least 200 MPa, at least 250 MPa, atleast 300 MPa, and/or no more than 1 GPa. In contemplated embodiments,glasses having a thickness of 0.5 mm or less have a compressive stressof at least 50 MPa, at least 80 MPa, at least 100 MPa, at least 150 MPa,at least 175 MPa, at least 200 MPa, at least 250 MPa, and/or no morethan 1 GPa.

In some embodiments, the thermally induced central tension may begreater than 40 MPa, or greater than 50 MPa, or greater than 75 MPa, orgreater than 100 MPa. In other embodiments, the thermally inducedcentral tension may be less than 300 MPa, or less than 400 MPa. Thethermally induced central tension may be from about 50 MPa to about 300MPa, about 60 MPa to about 200 MPa, about 70 MPa to about 150 MPa, orabout 80 MPa to about 140 MPa.

If sufficient energy is stored in the region of tensile stress 550, theglass will break like safety glass or “dice” when sufficiently damaged.As used herein, a glass sheet is considered to dice when an area of theglass sheet 25 cm² breaks into 40 or more pieces. In some embodiments,dicing is used as a qualitative measure of showing that the glass sheetis “fully tempered” (i.e., for 2 mm or thicker glass, where the glasssheet has a compressive stress of at least 65 MPa or an edge compressionof at least 67 MPa).

Another aspect comprises thermally strengthened glass sheets having highfictive temperatures and increased damage resistance. Surface fictivetemperatures may be determined by any suitable method, includingdifferential scanning calorimetry, Brillouin spectroscopy, or Ramanspectroscopy.

In some methods of determining surface fictive temperatures, it may benecessary to break the glass to relieve the “temper stresses” induced bythe heat strengthening process in order to measure fictive temperaturewith reasonably accuracy. It is well known that characteristic structurebands measured by Raman spectroscopy shift in a controlled manner bothwith respect to the fictive temperature and with respect to appliedstress in silicate glasses. This shift can be used to non-destructivelymeasure the fictive temperature of a thermally strengthened glass sheetif the temper stress is known.

Stress effects on the Raman spectrum of silica glass are reported in D.R. Tallant, T. A. Michalske, and W. L. Smith, “The effects of tensilestress on the Raman spectrum of silica glass,” J. Non-Cryst. Solids, 106380-383 (1988). Commercial glasses of 65 wt % silica or more havesubstantially the same response. Although the reported stress responseis for uniaxial stress, in the case of a unibiaxial stress state such asthat which is observed in tempered glass, σ_(xx)=σ_(yy), the peak can beexpected to shift by twice that expected by a uniaxial stress. The peaknear 1090 cm⁻¹ in soda-lime glass and in glass 2 corresponds to the 1050cm⁻¹ peak observed in silica glass. The effects of stress on the 1050peak in silica, and on the corresponding peak in SLG and other silicateglasses can be expressed, as a function of stress a in MPa, by a)ω(cm⁻¹)=1054.93−0.00232·σ.

A calibration curve was produced of Raman band position as a function ofthe fictive temperature for SLG and another glass, glass 2. Glasssamples were heat-treated for various times, 2-3 times longer than thestructural relaxation times calculated by τ=10*η/G, where η is theviscosity, and G the shear modulus. After heat-treatment the glasseswere quenched in water to freeze the fictive temperature at theheat-treatment temperature. The glass surfaces were then measured bymicro Raman at 50× magnification and a 1-2 μm spot size using a 442 nmlaser, 10-30 s exposure time, and 100% power, over the range of 200-1800cm⁻¹. The position of the peak at 1000-1200 cm⁻¹ was fit using computersoftware, Renishaw WIRE version 4.1 in this case. A good fit of the 1090cm⁻¹ Raman peak measured in SLG on the air side as a function of fictivetemperature Tf (in ° C.) is given by b) ω(cm⁻¹)=1110.66−0.0282·Tf. Forglass 2, a good fit is given by c) ω(cm⁻¹)=1102.00·0.0231·Tf.

Using the relationships established in equations a), b), and c), it ispossible to express the fictive temperature of the glass as a functionof a measured Raman peak position with a correction factor due tosurface compressive stress. A compressive stress of 100 MPa, σ_(c),shifts the Raman band position equivalent to approximately a 15 to 20degree Celsius reduction in the fictive temperature. The followingequation is applicable to SLG:

$\begin{matrix}{{T_{f}\left( {{^\circ}\mspace{14mu}{C.}} \right)} = {\left\lbrack \frac{{\omega\left( {cm}^{- 1} \right)} - {1110.66\left( {cm}^{- 1} \right)}}{{- 0.0282}\left( \frac{{cm}^{- 1}}{{^\circ}\mspace{14mu}{C.}} \right)} \right\rbrack + {2\left\lbrack {0.082*{\sigma_{C}({MPa})}} \right\rbrack}}} & (1)\end{matrix}$The equation applicable to glass 2 is:

$\begin{matrix}{{T_{f}\left( {{^\circ}\mspace{14mu}{C.}} \right)} = {\left\lbrack \frac{{\omega\left( {cm}^{- 1} \right)} - {1102\left( {cm}^{- 1} \right)}}{{- 0.0231}\left( \frac{{cm}^{- 1}}{{^\circ}\mspace{14mu}{C.}} \right)} \right\rbrack + {2\left\lbrack {0.0096*{\sigma_{C}({MPa})}} \right\rbrack}}} & (2)\end{matrix}$In these equations, ω is the measured peak wavenumber for the peak near1090 cm⁻¹, σ_(c) is the surface compressive stress measured by anysuitable technique, yielding stress-corrected measurement of fictivetemperature in ° C.

As a demonstration of increased damage resistance, four glass sheetsamples were prepared, two 6 mm soda lime glass (SLG) sheets byconventional tempering methods to approximately 70 and 110 MPa surfacecompressive stress (CS), and two 1.1 mm SLG by the methods andapparatuses disclosed herein to about the same levels of CS. Twoadditional sheets, one of each thickness were used as controls. Thesurfaces of each test sheet were subjected to standard Vickersindentation. Various levels of force were applied, for 15 seconds each,and after a 24 hour wait, indentations were each examined. As shown inTable I, the 50% cracking threshold (defined as the load at which theaverage number of cracks appearing is two out of the four points of theindenter at which cracks tend to initiate) was determined for eachsample.

The table shows that the Vickers crack initiation threshold for SLGprocessed by conventional convective gas tempering (as reflected in the6 mm sheet) is essentially the same as that for annealed or as-deliveredSLG sheets, rising from between zero and one Newton to about one to lessthan two Newtons. This correlates with the relatively modest rise insurface fictive temperature (T_(fs) or Tf_(surface)) of ˜25 to 35° C.relative to glass transition temperature (T_(g)=550° C. for SLG, definedas η=10^(12-13.3) Poise) that was provided by conventional tempering. Incontrast, by tempering using the present methods and apparatuses, theVickers crack initiation threshold improved to greater than 10 N, a10-fold increase over the Vickers damage resistance imparted byconventional tempering. In the embodied glasses, the T_(fs) minus T_(g)was at least 50° C., or at least 75° C., or at least 90° C., or in therange of from approximately 75° C. to 100° C. Even in embodimentscomprising lower levels of heat strengthening, the embodied glasses canstill provide increased resistance, at levels such as 5 N, for instance.In certain contemplated embodiments, the 50% cracking threshold after a15 second Vickers crack initiation test may be equal to or greater than5 N, 10 N, 20 N, or 30 N.

TABLE I Cracking Thickness CS Surface T_(f) Threshold Sample (mm) (MPa)(° C.) (N) Control 1.1 Annealed ~T_(g) (550) 0-1 Control 6 Annealed~T_(g) (550) 0-1 Thin low strength 1.1 −72 626 10-20 Thick low strength6 −66 575 1-2 Thin medium strength 1.1 −106 642 10-20 Thick mediumstrength 6 −114 586 1-2

The following non-dimensional fictive temperature parameter θ can beused to compare the relative performance of a thermal strengtheningprocess in terms of the fictive temperature produced. Given in terms ofsurface fictive temperature θs in this case:θs=(T _(fs) −T _(anneal))/(T _(soft) −T _(anneal))  (3)where T_(fs) is the surface fictive temperature, T_(anneal) (thetemperature of the glass at a viscosity of η=10^(13.2) Poise) is theannealing point and T_(soft) (the temperature of the glass at aviscosity of η=10^(7.6) Poise) is the softening point of the glass ofthe sheet. FIG. 5 is a plot of θs for measured surface fictivetemperatures as a function of heat transfer rate, h, applied duringthermal strengthening for two different glasses. As shown in the figure,the results for the two different glasses overlie each other fairlyclosely. This means that parameter θ provides a means to compare thefictive temperatures of different glasses compared directly, in relationto the heat transfer rate h required to produce them. The vertical rangeof results at each h corresponds to variation in the value of T₀, theinitial temperature at the start of quenching. In embodiments, parameterθs comprises from about 0.2 to about 0.9, or about 0.21 to about 0.09,or about 0.22 to about 0.09, or about 0.23 to about 0.09, or about 0.24to about 0.09, or about 0.25 to about 0.09, or about 0.30 to about 0.09,or about 0.40 to about 0.09, or about 0.5 to about 0.9, or about 0.51 toabout 0.9, or about 0.52 to about 0.9, or about 0.53 to about 0.9, orabout 0.54 to about 0.9, or about 0.54 to about 0.9, or about 0.55 toabout 0.9, or about 0.6 to about 0.9, or even about 0.65 to about 0.9.

Another aspect comprises a thermally strengthened glass sheet having ahigh temperability and/or heat transfer value. The “specific thermalstress” of a glass is given by:

$\begin{matrix}\frac{\alpha \cdot E}{1 - \mu} & (4)\end{matrix}$where α is the (low temperature linear) CTE of the glass, E is themodulus of elasticity and μ is Poisson's ratio. This value is used toindicate the level of stress produced within a given glass compositionwhen subjected to a temperature gradient. It may also be used as anestimator of thermal “temperability.” At higher thermal transfer rates(such as at about 800 W/m²K and above, for example), however, the hightemperature or “liquidus” CTE of the glass begins to affect temperingperformance, therefore, under such conditions, the temperabilityparameter Ψ, based on an approximation of integration over the changingCTE values across the viscosity curve, is found to be useful:Ψ=E·[T _(strain)·α_(CTE) ^(S)+α_(CTE) ^(L)·(T _(soft) −T_(strain))]  (5)where α^(S) _(CTE) is the low temperature linear CTE (equivalent to theaverage linear expansion coefficient from 0-300° C. for the glass),expressed in 1/° C. (° C.⁻¹), α^(L) _(CTE) is the high temperaturelinear CTE (equivalent to the high-temperature plateau value which isobserved to occur somewhere between the glass transition and softeningpoint an elastic modulus), expressed in 1/° C. (° C.⁻¹), E is theelastic modulus of the glass, expressed in GPa (not MPa) (which allowsvalues of the (non-dimensional) parameter Ψ to range generally between 0and 1), T_(strain) is the strain point temperature of the glass, (thetemperature of the glass at a viscosity of η=10^(14.7) Poise) expressedin ° C., and T_(soft) is the softening point of the glass (thetemperature of the glass at a viscosity of η=10^(7.6) Poise), expressedin ° C.

The thermal strengthening process and resulting surface compressivestresses were modeled for glasses having varying properties to determinethe tempering parameter, Ψ. The glasses were modeled at the samestarting viscosity of 10^(8.2) Poise and at varying heat transfercoefficients. The properties of the various glasses are shown in TableII, together with the temperature for each glass at 10^(8.2) Poise andthe calculated value of the temperability parameter Ψ for each.

TABLE II Strain CTE CTE 10^(8.2) Poise Softening Point Glass Modulus lowhigh ° C. Point ° C. ° C. ψ SLG 72 8.8 27.61 705 728 507 0.76 2 73.38.53 20.49 813 837 553 0.77 3 65.5 8.26 26 821 862 549 0.83 4 65 8.6920.2 864 912 608 0.74 5 63.9 10.61 22 849 884 557 0.84 6 58.26 3.5 20.2842 876 557 0.49 7 73.6 3.6 13.3 929 963 708 0.44 8 81.1 3.86 12.13 968995 749 0.48

The results in Table III show that Ψ is proportional to the thermalstrengthening performance of the glass. This correlation is furthershown in FIG. 6, which provides an embodied example for a high heattransfer rate (a heat transfer coefficient of 2093 W/m²K (0.05cal/s·cm²·° C.)) and a glass sheet thickness of only 1 mm. As seen inthe figure, the variation in the seven differing glasses' resultingcompressive stress correlates well with the variation in the proposedtemperability parameter Ψ.

In another aspect, it has been found that for any glass, at any givenvalue of the heat transfer coefficient, h (expressed in cal/cm²-s-° C.),the curves of surface compressive stress (σ_(CS), in MPa) vs. thickness(t, in mm) can be fit (over the range of t from 0 to 6 mm) by thehyperbola, where P₁ and P₂ are functions of h such that:

$\begin{matrix}{{\sigma_{CS}\left( {{Glass},h,t} \right)} = {{{C\left( {h,t} \right)}*{\Psi({Glass})}} = {\frac{{P_{1}(h)}*t}{\left( {{P_{2}(h)} + t} \right)}*{\Psi({Glass})}}}} & (6)\end{matrix}$or with the expression for Ψ substituted in, the curve of compressivestress σ_(CS) (Glass, h, t) is given by:

$\begin{matrix}{\frac{{P_{1}(h)}*t}{\left( {{P_{2}(h)} + t} \right)} \cdot E \cdot \left\lbrack {{T_{strain} \cdot \alpha_{CTE}^{s}} + {\alpha_{CTE}^{L} \cdot \left( {T_{soft} - T_{strain}} \right)}} \right\rbrack} & (7)\end{matrix}$where the constants P₁, P₂, in either (6) or (7) above, are eachcontinuous functions of the heat transfer value, h, given by:

$\begin{matrix}{{P_{1} = {910.2 - {259.2 \cdot {\exp\left( {- \frac{h}{0.143}} \right)}}}}{and}} & (8) \\{P_{2} = {2.53 + \frac{23.65}{\left( {1 + \left( \frac{h}{0.00738} \right)^{1.58}} \right)}}} & (9)\end{matrix}$The constants P₁, P₂, are graphed as functions of h in FIGS. 7 and 8,respectively. Accordingly, by using a value of P₁, for a given h and thecorresponding P₂, for that same h in expression (6) or (7) above, acurve is specified corresponding to the surface compressive stress (CS)obtainable at that h, as a function of thickness t.

In some embodiments, a similar expression may be used to predict thecentral tension (CT) of a thermally strengthened glass sheet,particularly at a thickness of 6 mm and less, and the thermal transfercoefficient, such as 800 W/m²K and up, by simply dividing thecompressive stress predicted under the same conductions by 2. Thus,expected central tension may be given by

$\begin{matrix}{\frac{{P_{1\;{CT}}\left( h_{CT} \right)}*t}{\left( {{P_{2{CT}}\left( h_{CT} \right)} + t} \right)} \cdot E \cdot \left\lbrack {{T_{strain} \cdot \alpha_{CTE}^{s}} + {\alpha_{CTE}^{L} \cdot \left( {T_{soft} - T_{strain}} \right)}} \right\rbrack} & (10)\end{matrix}$Where P_(1CT) and P_(2CT) are given as follows:

$\begin{matrix}{{P_{1\;{CT}} = {910.2 - {259.2 \cdot {\exp\left( {- \frac{h_{CT}}{0.143}} \right)}}}}{and}} & (11) \\{P_{2{CT}} = {2.53 + \frac{23.65}{\left( {1 + \left( \frac{h_{CT}}{0.00738} \right)^{1.58}} \right)}}} & (12)\end{matrix}$In some embodiments, h and h_(CT), may have the same value for a givenphysical instance of thermal strengthening. However, in someembodiments, they may vary and providing separate variables and allowingvariation between them allows for capturing within descriptiveperformance curves instances in which the typical ratio of 2:1 CS/CTdoes not hold.

One or more embodiments of the currently disclosed processes andapparatuses have produced thermally strengthened SLG sheets at all ofthe heat transfer rate values (h and h_(CT)) shown in Table III.

TABLE III Table IV(h and h_(CT) values according to exemplaryembodiments) cal/s · cm² · ° C. W/m²K 0.010 418.68 0.013 544.284 0.018753.624 0.019 795.492 0.020 837.36 0.021 879.228 0.022 921.096 0.023962.964 0.027 1130.436 0.028 1172.304 0.029 1214.172 0.030 1256.04 0.0311297.908 0.033 1381.644 0.034 1423.512 0.038 1590.984 0.040 1674.720.041 1716.588 0.042 1758.456 0.045 1884.06 0.047 1967.796 0.0482009.664 0.049 2051.532 0.050 2093.4 0.051 2135.268 0.052 2177.136 0.0532219.004 0.054 2260.872 0.055 2302.74 0.060 2512.08 0.061 2553.948 0.0622595.816 0.063 2637.684 0.065 2721.42 0.067 2805.156 0.069 2888.8920.070 2930.76 0.071 2972.628 0.078 3265.704 0.080 3349.44 0.081 3391.3080.082 3433.176 0.095 3977.46 0.096 4019.328 0.102 4270.536 0.1044354.272 0.105 4396.14 0.127 5317.236 0.144 6028.992 0.148 6196.4640.149 6238.332 0.184 7703.712

In some embodiments, the heat transfer value rates (h and h_(CT)) may befrom about 0.024 to about 0.15, about 0.026 to about 0.10, or about0.026 to about 0.075 cal/s·cm²·° C.

FIG. 9 shows the newly opened performance space in MPa of surfacecompression of a glass sheet as a function of thickness t (in mm), by agraph of C(h,t)·Ψ(SLG) for selected values of h according to equations6-9 above, with Ψ(SLG) corresponding to the value of Ψ for SLG in TableIII. The traces labeled GC represent the estimated range of maximumstresses versus thinness of SLG sheets achievable by gas convectivetempering, from 0.02 cal/s·cm²·° C. (or 840 W/m²K) to 0.03 cal/s·cm²·°C. or 1250 W/m²K, assuming that these levels of heat transfercoefficient can be employed in that process at a heated glass viscosityof 10^(8.2) Poises or about 704° C., a temperature above the capabilityof convective gas processes.

Examples of highest reported sheet CS values based on gas convectivetempering processes are shown by the triangle markers labeled Gas in thelegend. The value 601 represents advertised product performancecapability of commercial equipment, while the value 602 is based on anoral report at a glass processing conference. The trace labeled LCrepresents the curve of maximum stresses versus thinness of SLG sheetsestimated to be achievable by liquid contact tempering, given by a heattransfer rate h of 0.0625 cal/s·cm²·° C. (or about 2600 W/m²K), alsoassuming processing at an initial heated glass viscosity of 10^(8.2)Poises or about 704° C. Examples of highest reported sheet CS valuesbased on liquid contact tempering processes are shown by the circlemarkers labeled Liquid in the legend. The higher of the two values at 2mm thickness is based on a report of tempering of a borosilicate glasssheet, and the stress achieved has been scaled for the figure by(Ψ_(SLG))/(Ψ_(borosilicate)) for scaled direct comparison.

The trace labeled 704 represents stresses achievable by one or moreembodiments of the presently disclosed methods and apparatuses at a heattransfer rate of 0.20 cal/s·cm²·° C. (or about 8370 W/m²K) and aninitial temperature, just before quenching, of 704° C. The level ofstress on the glass sheet thus achievable represents almost the samescope of improvement over liquid tempering strength levels as liquidtempering represents over state of the art gas convective tempering. Butthe 704 boundary is not an upper limit—embodiments have been shown to beviable above this value due to the good control of form and flatnessachievable in a small-gap gas bearing thermal strengthening at evenhigher temperatures (at lower viscosities of the glass). The tracelabeled 730 shows some of the additional strengthening performanceachieved by a heat transfer rate of 0.20 cal/s·cm²·° C. (or about 8370W/m²K) at a starting temperature for a SLG sheet of 730° C., very nearor above the softening point of the glass. Significant improvements incompressive stress and thus in glass sheet strength are thus achievedparticularly by the combination of high heat transfer rate and the useof high initial temperatures enabled by the good handling and control ofsheet flatness and form in a tight gas bearing and the improvements areparticularly striking at thickness 2 mm and below.

FIG. 10 shows the traces of FIG. 9 explained above, at 2 mm and below,but with compressive stress as a function of thickness plotted forselected examples of tempered glass sheets produced by one or moreembodiments of the present disclosure, showing the extreme combinationof thermal strengthening levels and thinness enabled by the presentdisclosure.

In another embodiment, thermally strengthened glass sheet disclosedherein has both high thermal stresses and low, as-formed surfaceroughness. The processes and methods disclosed herein can thermallystrengthen a sheet of glass without increasing the surface roughness ofthe as-formed surfaces. For example, incoming float glass air-sidesurfaces, and incoming fusion formed glass surfaces, were characterizedby atomic force microscopy (AFM) before and after processing. R_(a)surface roughness was less than 1 nm (0.6-0.7 nm) for incoming 1.1 mmsoda lime float glass and was not increased by thermal strengtheningaccording to the present processes. Similarly, a R_(a) surface roughnessof less than 0.3 nm (0.2-0.3) for 1.1 mm sheets of fusion formed glasswas maintained by thermal strengthening according to this disclosure.Accordingly, thermally strengthened glass sheets have a surfaceroughness on a least a first surface in the range of from 0.2 to 1.5 nmR_(a) roughness, 0.2 to 0.7 nm, 0.2 to 0.4 or even such as 0.2 to 0.3nm, over at least an area of 10×10 μm. Surface roughness may be measuredover an area of 10×10 μm in exemplary embodiments, or in someembodiments, 15×15 μm.

In some embodiments, the glass sheet has one or more coatings that areplaced on the glass prior to the thermal strengthening of the glasssheet. The processes herein can be used to produce a strengthened glasssheets having one or more coatings, wherein the coating is placed on theglass prior to thermal strengthening and is unaffected by the process.Specific coatings that are advantageously preserved on glass sheets ofthe present disclosure include low E coatings, reflective coatings,antireflective coatings, anti-fingerprint coatings, cut-off filters,pyrolytic coatings, etc.

In another embodiment, the thermally strengthened glass sheets describedherein have high flatness. Controlled gas bearings are preferably usedin transporting and heating, and in some embodiments, can be used toassist in controlling and/or improving the flatness of the glass sheet,resulting in higher degree of flatness than previously obtainable,particularly for thin and/or highly strengthened sheets. For example,sheets at least 0.6 mm can be strengthened with improvedpost-strengthening flatness. The flatness of thermally strengthenedglass sheets embodied herein can comprise 100 μm or less total indicatorrun-out (TIR) along any 50 mm length along one of the first or secondsurfaces thereof, 300 μm TIR or less within a 50 mm length on one of thefirst or second surfaces, 200 μm TIR or less, 100 μm TIR or less, or 70μm TIR or less within a 50 mm length on one of the first or secondsurfaces. In exemplary embodiments, flatness is measured along any 50 mmor less profile of the glass sheet. In contemplated embodiments, sheetswith thickness disclosed herein have flatness 200 μm TIR or less withina 20 mm length on one of the first or second surfaces, such as flatness100 μm TIR or less, flatness 70 μm TIR or less, flatness 50 μm TIR orless.

Embodiments of the methods and apparatuses have been applied to glasssheets having thickness ranging from 0.1 mm to 5.7 or 6.0 mm, including,in addition to the end point values, 0.2 mm, 0.28 mm, 0.4 mm, 0.5 mm,0.55 mm, 0.7 mm, 1 mm, 1.1 mm, 1.5 mm, 1.8 mm, 2 mm, and 3.2 mm.Contemplated embodiments include thermally strengthened glass sheetshaving thicknesses in ranges from 0.1 to 20 mm, from 0.1 to 16 mm, from0.1 to 12 mm, from 0.1 to 8 mm, from 0.1 to 6 mm, from 0.1 to 4 mm, from0.1 to 3 mm, from 0.1 to 2 mm, from 0.1 to less than 2 mm, from 0.1 to1.5 mm, from 0.1 to 1 mm, from 0.1 to 0.7 mm, from 0.1 to 0.5 mm andfrom 0.1 to 0.3 mm.

In some embodiments, thermally strengthened glass sheets have highaspect ratios—i.e., the length and width to thickness ratio is large.Because the processes used don't rely on high pressures or large volumesof air, flatness can be maintained during the process by the use of gasbearings and high aspect ratio glass sheets (i.e., glass sheets withhigh ratio of length to thickness, or of width to thickness) can bethermally strengthened while retaining the desired or necessary shape.Specifically, sheets with length to thickness and/or width to thicknessratios (“aspect ratios”) of approximately at least 10:1, at least 20:1,and up to and over 1000:1 can be strengthened. In contemplatedembodiments, sheets with aspect ratios of at least 200:1, at least500:1, at least 1000:1, at least 2000:1, at least 4000:1 can beprocessed.

Another aspect comprises thermally strengthened low coefficient ofthermal expansion (CTE) glass sheets. As noted above, thermalstrengthening effects are significantly dependent upon the CTE of theglass of which the glass sheet is comprised. However, thermalstrengthening of low CTE glasses may provide strengthened glasscompositions having advantageous properties, such as increased chemicalresistance, or better compatibility with electronic devices due to lowalkali content. Glass sheets having CTEs of 65, 60, 55, 50, 45, 40, andeven 35×10⁻⁶° C.⁻¹ and below are capable of safety-glass like breakpatterns (“dicing”) at thicknesses of less than 4 mm, less than 3.5 mm,less than 3 mm, and even at 2 mm or less. Glasses having CTE values of40×10⁻⁶° C.⁻¹ and below can be strengthened using the processesdescribed herein. Such glasses can have similar surface compressions toSLG sheets strengthened by convention commercial (gas convective)processes at the same thickness. In some embodiments, the compressivestress of low CTE glasses can comprise at least 50 MPa, at least 100MPa, at least 125 MPa, at least 150 MPa, at least 200 MPa, at least 250MPa, at least 300 MPa, or at least 400 MPa for glass sheets having athickness of no more than 1 cm, no more than 5 mm, no more than 3 mm, nomore 2 mm, no more than 1.5 mm, no more than 1 mm, no more than 0.75 mm,no more than 0.5 mm, no more than 0.3 mm, no more than 0.2 mm, or nomore than 0.1 mm.

Glass sheets formed according to the present disclosure have a multitudeof applications, for example in electronic devices, displays and inlaminates, such as glass-interlayer-glass laminates used in automotiveglass sidelights. Stronger and thinner laminates can be produced,resulting in weight and cost savings and fuel efficiency increases.Desirably, a thermally strengthened thin sheet may be cold bent andlaminated to a formed thicker glass, providing an easy and reliablemanufacturing process not requiring any hot forming of the thin sheet.

Process

In one aspect, an overall process for strengthening a glass sheetcomprises supporting or guiding at least a portion of a glass sheethaving a transition temperature, on a first surface, at least in part bya flow or a pressure of a gas delivered to a gap between the firstsurface and a first heat sink, the sheet temperature being above thetransition temperature of the glass, and then cooling the glass sheet bythermal conduction more than by convection. Conduction is a process ofheat transfer where energy is transmitted through interactions betweenadjacent molecules, while convection is a process of heat transfer whereenergy is communicated via motion of a fluid (e.g., air, helium, etc.),such as where heated fluid moves away from a heat source and is replacedby cooler fluid. In at least some embodiments, the terms “glass ceramic”or “ceramic” can be substituted and/or equally applied where the term“glass” is used.

In some embodiments, an overall process for strengthening a glass sheetcomprises heating a glass sheet in a hot zone and then cooling the glasssheet. The glass sheet has a transition temperature, which occurs iswhere the viscosity of the glass has a value of η=10¹²-10^(13.3) Poise.The glass is heated sufficiently to bring the glass sheet above thetransition temperature. Optionally, the glass can be transitioned fromthe hot zone to a cool zone through a transition zone. The surfaces ofthe glass sheet are positioned adjacent to heat sinks, one on eitherglass surface with a gap in between the glass surface and the heat sink.Gas is delivered into the gaps through multiple apertures in the heatsinks. The glass sheet is cooled by conduction more than by convectionand sufficiently to fix or create a thermally induced surfacecompression and a thermally induced central tension of the sheet.

An apparatus for enabling the processes described can include a heatingzone for heating a glass sheet to a temperature above the transitiontemperature and a cooling zone for cooling the heated glass sheet fromto provide a strengthened glass sheet. The apparatus can include anoptional transition zone between the heating zone and the cooling zone.The cooling zone can comprise a pair of gas bearings disposed onopposite sides of a gap, which can be configured to deliver a gas to thegap to cool the heated glass sheet by conduction more than byconvection. In some embodiments, the gas bearings can include aplurality of apertures for delivering the gas to the gap, and gasbearing surfaces that provide heat sinks capable of conducting heat awayfrom the heated glass sheet by conduction more than by convection.

One embodiment of a method according to this disclosure is illustratedin the flow chart of FIG. 11. The method or process 100 includes thestep 160 of supporting a glass sheet at least in part by a gas (throughgas flow and pressure as in some convective gas strengtheningprocesses). The sheet can be heated to above the its glass transitiontemperature while at the same time cooling the sheet: 1) by conductionmore than by convection through the gas to a heat sink, and 2)sufficiently to create or fix a thermally-induced surface compressionstress and a thermally-induced central tension stress, of the sheet whenat ambient temperature.

According to a variation on the embodiment of FIG. 11, depicted asmethod 100′ in the flow chart of FIG. 12, the method can include thestep 110 of heating a glass sheet sufficiently such that the sheet isabove a transition temperature of the glass. In step 130A the methodfurther includes positioning a first sheet surface facing a first heatsink surface across a first gap and, in step 130B, positioning thesecond sheet surface facing a second heat sink surface across a secondgap, the second heat sink surface. The heat sink surfaces can includeapertures and/or can be porous. The method 100 can further include, instep 160, cooling the sheet, by conduction more than by convectionthrough a gas to the respective heat sink surfaces, sufficiently tostrengthen the glass, that is, sufficiently to create or fix in thesheet a thermally-induced surface compression stress and athermally-induced central tension stress. The step 160 of cooling thesheet also can include delivering the gas to the first and second gapsthrough the apertures or porous heat sink. In some embodiments, the gasis delivered only through the apertures of the heat sink or only throughthe pores or pores and aperatures of the porous heat sink.

These and other related methods of this disclosure go against thecurrently dominant technique of gas-convection-cooling by usingconduction as the dominant mode of cooling, instead of convection.Instead of a solid-to-gas (glass to air) heat exchange, methodsdescribed herein incorporate a solid-to-solid (glass to heat sink) heatexchange, mediated across a small gap by a small amount of gas, both tobegin and to complete the cooling that produces thermal strengthening.Although some convection is present as the mediating gas flows into thesmall gap, warms, and leaves, conduction directly across the gap throughthe gas and into the heat sink is the principal mode of cooling. Unlikethe solid and liquid cooling methods described above, the conduction ismediated through a gas barrier layer. The use of a gas as anintermediate conductor, without contact of the sheet by liquid or solidmatter, can preserve the surface quality of the processed articles byavoiding contact other than by a gas. This can avoid the introduction ofunwanted distortions, spatial variation in strengthening andcontamination of the glass surfaces seen in liquid and solid cooling.The embodiments disclosed herein provide a unique, non-contact,conductive quench that allows for very high cooling rates that were notpreviously available in the art of thermal tempering.

Because conduction, ultimately solid-to-solid, allows for more rapidheat flow than convection, the cooling rate increases needed for thinnerglass sheets are not tied to gas velocity and volume. Gas flow and gapsize can, instead, be optimized for other purposes, according to variousembodiments and variations of the methods and apparatuses of the presentdisclosure, such as for stiffness of the gas cushion in the gap,supporting or for flattening and/or otherwise shaping a sheet,optimizing heat conduction, or simply maintaining sheet flatness and/orshape during thermal strengthening, as well as balancing ease of sheethandling with high cooling rates, for example. For example, heliumbecomes an economically viable alternative at low flow rates, and offersthermal conductivity about five times that of air.

Decreasing the volumes of air flowing over a glass sheet during coolingdecreases the potential risk of deformation of hot thin sheets by thehigh speed, high volume air flows otherwise required for strengtheningthin sheets, and allows softer, higher temperature sheets to be handledwith no or minimal distortion, further improving the achievable degreeof strengthening. Eliminating high air flow rates also eases problemssometimes seen in transporting the sheet into the quenching chamber(moving against the high air flow), and in keeping the high-flow coolerair from entering into and cooling the nearer parts of the furnace usedto heat the sheet.

Another advantage in the avoiding high air flow rates lies in the powerand energy savings achieved by using low gas flows and solid-gas-solidconduction. Points A and B of FIGS. 13A and 13B represent a high-endestimate of peak power use, per square meter of glass sheet, by acompressed air supply at relatively high flow. Practical low-end peakpower use of compressed air could be as little as 1/16 of the valuesshown. Points A and B do not include active cooling of the heat sink,however, which can be included in some embodiments, especially where amachine is in continuous, quasi-continuous or high frequency operation.

Referring again to FIGS. 13A and 13B, points A′ and B′ represent theconservatively estimated peak power levels for operation at points A andB when active cooling of the heat sink surfaces is factored in, assumingthe thermal load equivalent of a 300° C. drop in glass sheet temperatureis accomplished by an active cooling system having athermal-to-mechanical (or electrical) efficiency ratio of 7.5 to 1,within a time limit of 2.1 seconds for point A′ and within 1 second forpoint B′. (These points correspond approximately to glass sheetsactually tempered in the experimental apparatus described herein.)Although the four points within region R of FIGS. 13A and 13B illustrateto some degree the significance of the improvement obtainable by themethods and apparatuses of the present disclosure, it should be notedthat the full benefits are likely significantly understated in thefigures, because power demand is the quantity represented. For example,peak power of air blowers, as represented by the curve N, is notefficiently turned on and off, typically requiring gated airways toblock off large fans, which still rotate (but at reduced load), when airis not needed. Peak power demands of fluid cooling systems such aschilled water plants, represented by the points A′ and B′ as exampleseasily achievable according to the present disclosure, can generally bemuch more efficiently accommodated, and effective peak power would besignificantly lower, approaching A′ and B′ only as fully continuousoperation is approached. Thus, the difference in total energy demandswould tend to be greater than the difference for peak power demand,which is represented in the figure. In some embodiments, the processesdescribed herein have peak powers of less than 120 KW/m², less than 100KW/m², less than 80 KW/m² to thermally strengthen a glass sheet of 2 mmthickness or less.

The amount of conduction at conditions embodied in processes usingapparatuses described herein can be determined via the following. First,in the context of thermal strengthening by conduction as in the presentdisclosure, the thermal conductivity of the gas must be evaluated in thedirection of conduction, which is along a thermal slope. Air at hightemperature, at or near the surface of the sheet to be (or being)cooled, has significantly higher thermal conductivity than air at alower temperature such as air at or near room temperature (the nominalthermal conductivity of (dry) room temperature air (25° C.) isapproximately 0.026 W/m·K), at or near the surface of the heat sink. Anapproximation that assumes air over the whole gap to be at the averagetemperature of the two facing surfaces, at the start of cooling is used.A glass sheet may be at a temperature of 670° C., for example, while theheat sink surface may start at 30° C., for example. Accordingly, theaverage temperature of the air in the gap would be 350° C., at which dryair has a thermal conductivity of about 0.047 W/m·K; more than 75%higher than its thermal conductivity at room temperature andsufficiently high to conduct large amounts of heat energy through gapsof practical size as discussed below.

To illustrate, Q_(cond), the conductive component of the rate of heattransfer through a gap of distance g which gap has an area A_(g) (in adirection everywhere perpendicular to the direction of the gap distanceg) may be given by:

$\begin{matrix}{Q_{cond} = \frac{A_{g}{k\left( {T_{S} - T_{HS}} \right)}}{g}} & (13)\end{matrix}$where k is the thermal conductivity of the material (gas) in the gapevaluated in the direction of (or opposite of) heat conduction, T_(S) isthe temperature of the glass surface and T_(HS) is the temperature ofthe heat sink surface (or the heat source surface, for otherembodiments). As mentioned above, to evaluate k rigorously would requireintegrating the thermal conductivity of the gas along (or against) thedirection of conductive heat flow, as the thermal conductivity of thegas varies with temperature—but to good approximation, k may be taken asthe value of k for the gas in the gap when at the average of thetemperatures of the two surfaces, T_(S) and T_(HS).

Reframing equation (13) in units of heat transfer coefficient (units ofheat flow power per meter squared per degree Kelvin) gives:

$\begin{matrix}{\frac{Q_{cond}}{A_{g}\left( {T_{S} - T_{HS}} \right)} = \frac{k}{g}} & (14)\end{matrix}$so the effective heat transfer coefficient for conduction across the gapis the thermal conductivity of the medium in the gap (air in this case)(in units of W/mK) divided by the length of the gap (in meters), givinga value of Watts per meter squared per degree of temperature difference.

Table IV shows the heat transfer coefficients (k/g), due to conductionalone, for air and helium filled gaps, from 10 μm up to 200 μm in stepsof 10 μm each FIG. 14 (Prior Art) shows an industry-standard curve fromabout 35 years ago (with reference line at 2 mm added) showing the heattransfer coefficient required to fully temper a sheet of glass, as afunction of thickness in mm, under certain assumed conditions. As may beseen from a comparison of Table IV with FIG. 14, an air-filled gap ofapproximately 40 μm can allow full tempering of 2 mm thick glass byconduction. Using helium (or hydrogen, with similar thermalconductivity) as the gas, a gap of about 200 μm can be used to fullytemper 2 mm thick glass.

TABLE IV Air Helium conductivity (W/m/K) 0.047 conductivity (W/m/K)0.253 heat trans coeff. heat trans coeff. Gap (m) W/m²/K cal/s/cm² Gap(m) W/m²/K cal/s/cm² 0.00001 4700 0.11226 0.00001 25300 0.604291 0.000022350 0.05613 0.00002 12650 0.302145 0.00003 1566.67 0.03742 0.000038433.33 0.20143 0.00004 1175 0.028065 0.00004 6325 0.151073 0.00005 9400.022452 0.00005 5060 0.120858 0.00006 783.333 0.01871 0.00006 4216.670.100715 0.00007 671.429 0.016037 0.00007 3614.29 0.086327 0.00008 587.50.014032 0.00008 3162.5 0.075536 0.00009 522.222 0.012473 0.000092811.11 0.067143 0.0001 470 0.011226 0.0001 2530 0.060429 0.00011427.273 0.010205 0.00011 2300 0.054936 0.00012 391.667 0.009355 0.000122108.33 0.050358 0.00013 361.538 0.008635 0.00013 1946.15 0.0464840.00014 335.714 0.008019 0.00014 1807.14 0.043164 0.00015 313.3330.007484 0.00015 1686.67 0.040286 0.00016 293.75 0.007016 0.000161581.25 0.037768 0.00017 276.471 0.006604 0.00017 1488.24 0.0355470.00018 261.111 0.006237 0.00018 1405.56 0.033572 0.00019 247.3680.005908 0.00019 1331.58 0.031805 0.0002 235 0.005613 0.0002 12650.030215Using helium or hydrogen as the gas allows for a gap size about 5 timeslarger for the same heat transfer coefficient. In other words, usinghelium or hydrogen as the gas in the gap increases the heat transfercoefficient available for quenching by about 5 times at the same gapsize.

In addition to cooling through a gas by conduction more than byconvection, another embodiment includes heating (or heating and/orcooling) through a gas by conduction more than by convection. Regardingthe relative contributions of conduction and convection, whether forheating or cooling, the convective Q_(conv) component of the rate heattransfer across the gap (or gaps) may be given by:

$\begin{matrix}{Q_{conv} = {e\overset{.}{m}{C_{p}\left( {\frac{T_{S} + T_{HS}}{2} - T_{i}} \right)}}} & (15)\end{matrix}$where {dot over (m)} is the mass flow rate of the gas, Cp is thespecific heat capacity of the gas, T_(i) is the inlet temperature of thegas as it flows into the gap, and e is the effectiveness of the heatexchange between the gas flowing in the gap and the sheet surface andthe surface of the heat sink/source (the “walls” of the gap). The valueof e varies from 0 (representing zero surface-to-gas heat exchange) to 1(representing the gas fully reaching the temperature of the surfaces).The value of e can be computed by those skilled in the art of heattransfer using, for example, the e-NTU method.

Typically however, if the gap between the surface of the sheet and thesurface of the heat sink/source is small, the value of e will be verynearly equal to 1, meaning the gas heats nearly completely—to equal, onaverage, the average of the temperature of the two surfaces on eitherside—before it leaves the gap. Assuming e=1 (a slight overestimate ofthe rate of convective heat transfer), and the gas being supplied to thegap through the surface of the heat sink/source, it can be assumed thatthe initial temperature of the gas in the gap is the same as thetemperature of the surface of the heat sink/source (T_(i)=T_(HS)). Therate of heat transfer due to convection may then be simplified to:

$\begin{matrix}{Q_{conv} = {\overset{.}{m}{C_{p}\left( \frac{T_{S} - T_{HS}}{2} \right)}}} & (16)\end{matrix}$

To cool (or heat, assuming the amount of radiation from the heat sourcewhen heating is not too high) the sheet principally by conduction, inthe area of the gap, thus requires that:Q _(cond) >Q _(conv)  (17)Combining (17) with equations (13) and (16) gives the followingconditional:

$\begin{matrix}{\frac{k}{g} > \frac{\overset{.}{m}C_{p}}{2A_{g}}} & (18)\end{matrix}$which, when held, will essentially ensure that the sheet, in the area ofthe gap at issue, is cooled (or heated) principally by conduction.Accordingly, the mass flow rate in of the gas should be less than 2kA_(g)/gC_(p), or 2 k/gC_(p) per square meter of gap area. In anembodiment, {dot over (m)}<B·(2 kA_(g)/gC_(p)), where B is the ratio ofconvective cooling to conductive cooling. As used herein, B is apositive constant less than one and greater than zero. This ratio ofconvective cooling to conductive cooling can be any value from less thanone to 1×10⁻⁸. In some embodiments, B is less than 0.9, 0.8, 0.7, 0.6,0.5, 0.4, 0.1, 5×10⁻², 1×10⁻², 5×10⁻³, 1×10⁻³, 5×10⁻⁴, 1×10⁻⁴, 5×10⁻⁵,1×10⁻⁵, 5×10⁻⁶, 1×10⁻⁶, 5×10⁻⁷, 1×10⁻⁷, 5×10⁻⁸, or 1×10⁻⁸. In someembodiments, in is minimized, consistent with the needs of using the gasflow to support and control the sheet position relative to the heat sinksurface(s) Inn other embodiments, m should be selected to control theposition of the heat exchange surfaces themselves, relative to thesheet.

A diagrammatic cross-section of a glass sheet being cooled by conductionmore than by convection is shown in FIG. 15. A hot glass sheet 200 hasits first and second (major) surfaces 200 a, 200 b each facing arespective first and second surface 201 b, 202 b of respective first andsecond heat sinks 201 a, 202 a across respective gaps 204 a and 204 b.Gas 230 is fed through the first and second surfaces 201 b, 202 b asrepresented by the arrows, to supply the gaps 204 a, 204 b, and toassist in keeping the glass sheet centered or otherwise positionedbetween the heat sinks 201 a, 202 a. The air or other gas may leavepassing by the edges of the heat sinks 201 a, 202 a as shown by arrows240. By choosing the size of the gaps 204 a, 204 b and the gas and theflow rate of the gas 230 in accordance with the preceding paragraph andother discussion above, the glass sheet 200 will be cooled more byconduction than convection.

In some embodiments, the gaps 204 a, 204 b are configured to have athickness or distance across the gap sufficient such that the heatedglass sheet is cooled by conduction more than by convention. In someembodiments, gaps 204 a and 204 b may have a thicknesses of about 100 μmor greater (e.g., in the ranges from about 100 μm to about 200 μm, fromabout 100 μm to about 190 μm, from about 100 μm to about 180 μm, fromabout 100 μm to about 170 μm, from about 100 μm to about 160 μm, fromabout 100 μm to about 150 μm, from about 110 μm to about 200 μm, fromabout 120 μm to about 200 μm, from about 130 μm to about 200 μm, or fromabout 140 μm to about 200 μm). In other embodiments, gaps 204 a and 204b may have a thicknesses of about 100 μm or less (e.g., in the rangesfrom about 10 μm to about 100 μm, from about 20 μm to about 100 μm, fromabout 30 μm to about 100 μm, from about 40 μm to about 100 μm, fromabout 10 μm to about 90 μm, from about 10 μm to about 80 μm, from about10 μm to about 70 μm, from about 10 μm to about 60 μm, or from about 10μm to about 50 μm).

Heat sinks 201 a, 202 a may comprise solid or porous configurations.Suitable materials, include, but are not limited to aluminum, bronze,carbon or graphite, stainless steel, etc. Heat sink dimensions may bedesigned to be sufficient to address the size of the glass sheet and toefficiently and effectively transfer heat without changing the heat sinktemperature significantly. In the case where heat sinks 201 a and/or 202a are porous, they may still include additional apertures or holes forflowing gas or may use the porous structure to provide flow, or both. Insome embodiments, the heat sinks further comprise passages to allowfluid flow for controlling the temperature of the heat sink, describedin more detail in FIGS. 17A-17C and below.

Eliminating high gas flow rates of the prior art may enable use of verysmall apertures or pores in the heat sink face to provide the gas withinthe gap(s). In some embodiments, apertures may be less than 2 mm, lessthan 1.5 mm, less than 1 mm, less than 0.5 mm, less than 0.25 mm, orless than or equal to 200, 150, 100, 50, 30, 20, or 10 μm, when measuredin the smallest direction (e.g., diameter). In some embodiments, theapertures are from about 10 μm to about 1 mm, about 20 μm to about 1 mm,or about 50 μm to about 1 mm. Aperture spacing can be from about 10 μmto about 3 mm, about 20 μm to about 2 mm, or about 50 μm to about 1 mm,measured edge-to-edge of apertures. Small apertures or pores mayfunction as individual flow restrictors, providing high-performancegas-bearing-type dynamics, such as high levels of stiffness andconsistency of support of the sheet to position the sheet and controlgap size, allowing for high homogeneity of thermal strengthening effectsto avoid or reduce stress birefringence. Further, because very smallpores or apertures may be used, the relative amount of solid matter atthe surface of the heat sink facing the sheet surface across the gap(s)can be maximized, thereby increasing conductive heat flow. According toone embodiment, use of such apertures as the only path for providing gasto the gap(s) and configuring the apertures to lie in directions closeto normal to the heat sink surface can optimize gas-bearing-typedynamics, because the flow from the apertures may not be compromised bygas flows from, for example, additional larger apertures, from sourcesother than through the heat sink surface(s) adjacent to the sheet, or byother lateral flow.

FIGS. 16 and 17A-17C show an exemplary embodiment of an apparatus 300according to this disclosure. FIG. 16 show a schematic cross-sectionaldiagram of the apparatus 300 in which a glass sheet can be cooledthrough a gas into a conductive heat sink. The apparatus of includes ahot zone 310, a cold zone 330, and a transition gas bearing 320, bywhich a glass article may be moved from the hot zone 310 to the coldzone 330 such that no contact or substantially no contact occurs betweenthe glass and the bearings. The hot zone 310 has gas bearings 312 eachfed from a hot zone plenum 318, the bearings 312 having cartridgeheaters 314 inserted into holes through the bearings 312, which serve toheat the hot zone gas bearings 312 up to a desired starting processtemperature. A glass sheet (hot zone) 400 a is kept between the hot zonegas bearings 312 for a duration long enough to bring it to a desiredpre-cooling temperature.

In some embodiments, heating the sheet in the hot zone may be donepredominantly via conduction of heat from a heat sink through a thin gasbarrier. The conductive heating processes used in the hot zone can besimilar to, but the reverse of—i.e., pushing heat into the glasssheet—the cooling processes described above.

In some embodiments gaps 316 between the hot zone gas bearings 312 andthe glass sheet 400 a may be relatively large, on the order of 0.05″(1.27 mm) to 0.125″ (3.175 mm) or greater, since the glass sheet 400 amay be heated up relatively slowly and thermal radiation from the hotgas bearings 312 into the glass sheet 400 a is adequate for thispurpose. In other embodiments, hot zone gap values may be as small as150 microns per side or 500 microns per side. Smaller gaps may beadvantageous because they enable the bearings to have better“stiffness”—i.e., ability to centralize the glass and therefore flattenit while it is in its softened state. In some embodiments, the processmay re-form the glass sheets—flattening them—in the initial heatingstep. In some embodiments, the top and bottom hot zone bearings may beon actuators, allowing for changing the gap width in a continuous manneror, alternatively, allowing the glass to be brought into the hot zonewhen the gap is large and then compressing the gap to flattening theglass while it is still soft.

Process temperatures are dependent on a number of factors including theglass composition, glass thickness, glass properties (CTE, etc.), anddesired level of strengthening. Generally, the starting processtemperature may be any value between the glass transition temperatureand the Littleton softening point, or in some embodiments, even higher.For SLG, for example, a range process temperature may be from about 640to about 730° C. or about 690 to about 730° C. In some embodiments, theprocess temperature range can be from about 620 to about 800° C., about640 to about 770° C., about 660 to about 750° C., about 680 to about750° C., about 690 to about 740° C., or about 690 to about 730° C.

The glass sheet 400 a is heated to its desired starting processtemperature and it can then be moved from the hot zone 310 to the coldzone 330 using any suitable means. In some embodiments, moving the glasssheet 400 a from the hot zone 310 to the cold zone 330 may beaccomplished by, for example (1) tilting the entire assembly such thatgravity acting on the glass sheet forces it to move to the cold zone,(2) blocking off the gas flow from the leftmost exit of the hot zone 310(the sides are enclosed in this embodiment), thereby forcing all of thegas emanating from all of the gas bearings to exit from the rightmostexit of the cold zone, causing fluid forces to be exerted on the glasssheet 400 a and causing it to move to the cold zone 330, or (3) by acombination of (1) and (2)) The transition gas bearings 320 may besupplied with gas by transition bearing plenums 328. The solid materialthickness behind the surfaces of the transition gas bearings 320 may bethin and/or of low thermal mass and/or low thermal conductivity,allowing for reduced heat conduction from the hot zone 310 to the coldzone 330, which is fed by separate plenums 338. The transition gasbearings 320 may serve as a thermal break or transition between the twozones 310 and 330 and may serve to transition from the larger gaps 316of the hot zone down to small gaps 336 of the cold zone 330. Once theglass sheet (cold zone) 400 b moves into the cold zone 330 and into thechannel 330 a, it is stopped from exiting the right side exit by amechanical stop, not shown. Once the glass sheet 400 b coolssufficiently that the center has passed the glass transition (in thecase, for example, of 1 mm thick SLG, to below about 490° C.,corresponding in this example to about 325° C. at the surface), the stopgate may be removed and the glass sheet 400 b may be removed from theapparatus 300. If desired, the glass sheet 400 b may be left in the coldzone 330 until somewhere near room temperature before removal.

In the embodiment shown in FIG. 16, the cold zone 330 includes a channel330 a for receiving glass sheet 400 b (which is heated to a temperatureabove the glass transition temperature of the glass sheet in the hotzone) through an opening 330 b, conveying the glass sheet 400 b, andcooling the glass sheet 400 b in the cold zone. In one or moreembodiments, the channel 330 a includes a conveyance system that mayinclude gas bearings, roller wheels, conveyor belt, or other means tophysically transport the glass sheet through the cold zone. Becausecooling occurs essentially solid to solid, issues not present inconvection-dominated cooling may need to be addressed. For example, fortempering of a large thin sheet, the sheet is may be either (1)introduced quickly into the cold zone, optionally at higher speeds thanthose typically used in convection-based quenching or (2) the process isoperated in a quasi-continuous mode, in which multiple sheets are heatedand cooled one after the other in a continuous stream with little spacebetween them, and where the heat sink is actively cooled such that itreaches a thermal equilibrium so that the front and trailing edges ofthe large sheets have the similar thermal history.

In some embodiments, the cold zone 330 includes one or more heat sinks331 disposed adjacent to the channel 330 a. Where two heat sinks areutilized, such heat sinks may be disposed on opposite sides of thechannel 330 a, facing each other across a channel gap 330 a. In someembodiments, the heat sinks include a plurality of apertures 331 a whichform part of the gas bearing 332, and the surfaces of the cold gasbearings 332 of the cold zone 330 serve as the two heat sink surfaces.In some embodiments, the heat sinks and/or the surfaces thereof may besegmented. As noted above, in some embodiments, the heat sinks may beporous. In other embodiments, the heat sinks may be porous and theapertures are the pores of the porous heat sinks. The plurality ofapertures 332 b, a gas source and the channel gap 330 a may be in fluidcommunication. In some embodiments, the gas flows through the apertures331 a to form gas cushions in the channel gap 330 a. The gas cushions ofsome embodiments prevent the glass sheet 400 b from contacting the heatsink 331 surfaces. The gas also serves as the gas through which theglass sheet 400 b is cooled by conduction more than by convection. Insome embodiments, the gas flowed through the apertures cools the heatsinks. In some embodiments, the gas flowed through the apertures bothcools the glass by conduction, across the gap into the heat sinks, morethan by convention, and cools the heat sinks 331. In some instances, aseparate gas or fluid may be used to cool the heat sinks 331. Forinstance, the heat sinks 331 may include passages 334 for flowing acooling fluid there through to cool the heat sinks 331, as is more fullydescribed with respect to FIG. 17A. The passages 334 can be enclosed.

Where two heat sinks are used (i.e., a first heat sink and the secondheat sink), one or more gas sources may be used to provide a gas to thechannel gap 330 a. The gas sources may include the same gas as oneanother or different gases. The channel gap 330 a may, therefore,include one gas or a mixture of gases from different gas sources or thesame gas source. Exemplary gases include air, nitrogen, carbon dioxide,helium or other noble gases, hydrogen and various combinations thereof.The gas may be described by its thermal conductivity when it enters thechannel 330 a just before it begins to conductively cool the glass sheet400 b. In some instances, the gas may have a thermal conductivity ofabout 0.02 W/(m·K) or greater, about 0.025 W/(m·K) or greater, about0.03 W/(m·K) or greater, about 0.035 W/(m·K) or greater, about 0.04W/(m·K) or greater about 0.045 W/(m·K) or greater, about 0.05 W/(m·K) orgreater, about 0.06 W/(m·K) or greater, about 0.07 W/(m·K) or greater,about 0.08 W/(m·K) or greater, about 0.09 W/(m·K) or greater, about 0.1W/(m·K) or greater, about 0.15 W/(m·K) or greater, or about 0.2 W/(m·K)or greater).

The processes described allow for high heat transfer rates. Using air asthe gas, heat transfer rates as high as 350, 450, 550, 650, 750, 1000,and 1200 kW/m² or more are possible through conduction alone. Usinghelium or hydrogen, heat transfer rates of 5000 kW/m² or more can beachieved.

The heat sinks 331 of one or more embodiments may be stationary or maybe movable to modify the thickness of the channel gap 330 a. Thethickness of the glass sheet 400 b may be in a range from about 0.4times the thickness to about 0.6 times the thickness of channel gap 300a, which is defined as the distance between the facing surfaces of theheat sinks 331. In some instances, the channel gap is configured to havea thickness sufficient such that the heated glass sheet is cooled byconduction more than by convection. In some embodiments, the channel gapmay have a thickness such that when glass sheet 400 b is being conveyedthrough the channel, the distance between the glass sheet and the heatsink surface (the gap) is about 100 μm or greater (e.g., in the rangefrom about 100 μm to about 200 μm, from about 100 μm to about 190 μm,from about 100 μm to about 180 μm, from about 100 μm to about 170 μm,from about 100 μm to about 160 μm, from about 100 μm to about 150 μm,from about 110 μm to about 200 μm, from about 120 μm to about 200 μm,from about 130 μm to about 200 μm, or from about 140 μm to about 200μm). In some embodiments, the channel gap may have a thickness such thatwhen glass sheet 400 b is being conveyed through the channel, thedistance between the glass sheet and the heat sink surface (the gap orgaps 336) is about 100 μm or less (e.g., in the range from about 10 μmto about 100 μm, from about 20 μm to about 100 μm, from about 30 μm toabout 100 μm, from about 40 μm to about 100 μm, from about 10 μm toabout 90 μm, from about 10 μm to about 80 μm, from about 10 μm to about70 μm, from about 10 μm to about 60 μm, or from about 10 μm to about 50μm). The total thickness of the channel gap 330 a is dependent on thethickness of the glass sheet 400 b but can be generally characterized as2 times the distance between the heat sink surface and the glass sheetplus the thickness of the glass sheet. In some embodiments, the distanceor gaps 336 between the glass sheet and the heat sinks may not be equal.In such embodiments, the total thickness of the channel gap 330 a may becharacterized as the sum of the distances between the glass sheet andeach heat sink surface and the thickness of the glass sheet.

In some instances, the total thickness of the channel gap may be lessthan about 2500 μm (e.g., in the range from about 120 μm to about 2500μm, about 150 μm to about 2500 μm, about 200 μm to about 2500 μm, about300 μm to about 2500 μm, about 400 μm to about 2500 μm, about 500 μm toabout 2500 μm, about 600 μm to about 2500 μm, about 700 μm to about 2500μm, about 800 μm to about 2500 μm, about 900 μm to about 2500 μm, about1000 μm to about 2500 μm, about 120 μm to about 2250 μm, about 120 μm toabout 2000 μm, about 120 μm to about 1800 μm, about 120 μm to about 1600μm, about 120 μm to about 1500 μm, about 120 μm to about 1400 μm, about120 μm to about 1300 μm, about 120 μm to about 1200 μm, or about 120 μmto about 1000 μm). In some instances, the total thickness of the channelgap may be about 2500 μm or more (e.g., in the range from about 2500 μmto about 10,000 μm, about 2500 μm to about 9,000 μm, about 2500 μm toabout 8,000 μm, about 2500 μm to about 7,000 μm, about 2500 μm to about6,000 μm, about 2500 μm to about 5,000 μm, about 2500 μm to about 4,000μm, about 2750 μm to about 10,000 μm, about 3000 μm to about 10,000 μm,about 3500 μm to about 10,000 μm, about 4000 μm to about 10,000 μm,about 4500 μm to about 10,000 μm, or about 5000 μm to about 10,000 μm).

The apertures 331 a in the heat sink 331 may be positioned to beperpendicular to the heat sink surface or may be positioned at an angleof 20 degrees or less (e.g., about 15 degrees or less, about 10 degreesor less or about 5 degrees or less) from perpendicular to the heat sinksurface.

In some embodiments, the material behind the heat sink (cold bearing332) surfaces can be any suitable material having high heat transferrates, including metals e.g. stainless steel, copper, aluminum),ceramics, carbon, etc.). This material may be relatively thick comparedto the material behind the surfaces of the transition bearings 320, asshown in the figure, such that heat sink can easily accept relativelylarge amounts of thermal energy. FIG. 17A is a cut-away perspectivecross section of an apparatus similar to that of FIG. 16, albeitreversed from right to left, and comprising additionally a load/unloadzone 340 next to the cold zone 330 of the apparatus 300, including aload/unload gas bearing 342 with a glass sheet 400 c positioned thereon.Also, the apparatus of FIG. 17A uses tight channel gaps (not indicatedon the figure) in all of the hot, transition bearing, and cold zones310, 320, and 330, respectively.

The inset in FIG. 17A shows an alternative embodiment of a cold zone gasbearing 332 a, in which the gas bearing 322 a is actively cooled bycoolant channels 334 between gas bearing feed holes 333, where the feedholes feed the apertures in the surface of the bearing 322 a. Thecooling channels 334 are defined between heat sink segments 333 b whichare assembled together to form the heat sink 332 a and the surfacethereof facing the glass sheet 400 b. The cooling channels 334 may bepositioned very near the surface of the heat sink 331 in the solidmaterial of the gas bearing 332, with a region of solid bearing materialbetween the heat sink/gas bearing surface and the nearest-the-surfaceedge of the coolant channel 334 having the same width as thenearest-the-surface edge of the coolant channel 334. Accordingly, insome embodiments there is no region of reduced cross section in thesolid material of the heat sink 331/gas bearing 332 a between a coolantchannel 334 and the surface facing the glass 400 b. This differs fromthe typical convective gas cooling equipment, because the high gas flowrates mandate that significant space be provided in the middle of thearray of gas nozzles for the gas flows to escape. Where active coolingis used, typically it is necessary to have a region of reduced crosssection in the solid material of the gas nozzle design, relative to thesolid material nearest the glass surface. The reduced cross sectionregion is generally positioned between the active cooling fluid andglass sheet under treatment, in order to give a high-volume path for thelarge volume of heated gas returning from the sheet.

FIG. 17B shows yet another alternative embodiment of a cold zone gasbearing 332 b like that of the inset of FIG. 17A. In this embodiment,coolant channels 334 are formed between a gas bearing feed member 335,containing gas bearing feed holes 333, and a gas bearing face member 337a which provides the glass sheet 400 b facing surface of the gas bearing332 b. FIG. 17C shows yet another alternative cold zone gas bearing 332c, similar structure to the embodiment of FIG. 17B, but having a porousmember 339 between a bearing plate member 337 b, which porous member 339forms the surface facing the glass sheet 400 b.

The processes and apparatuses described herein may generally be usedwith almost any glass composition, and some embodiments can be used withglass laminates, glass ceramics, and/or ceramics. In embodiments, theprocesses can be used with glass compositions having high CTEs. Inembodiments, glasses used include alkali aluminosilicates, such asCorning's® Gorilla® Glasses, SLG, soda- or alkali-free glasses and thelike. In some embodiments, the glasses used have CTEs of greater thanabout 40×10⁻⁷/° C., greater than about 50×10⁻⁷/° C., greater than about60×10⁻⁷/° C., greater than about 70×10⁻⁷/° C., greater than about80×10⁻⁷/° C., or greater than about 90×10⁻⁷/° C.

The processes and apparatuses described herein may generally be usedwith glasses of any thickness. In some embodiments glass sheets of 3 mmor less in thickness are used. In some embodiments, the glass thicknessis about 8 mm or less, about 6 mm or less, about 3 mm or less, about 2.5min or less, about 2 mm or less about 1.8 mm or less, about 1.6 mm orless, about 1.4 min or less, about 1.2 mm or less, about 1 mm or less,about 0.8 mm or less, about 0.7 mm or less, about 0.6 mm or less, about0.5 mm or less, about 0.4 mm or less, about 0.3 mm or less, or about0.28 or less. In some embodiments, the glass is a flexible glass sheet.In other embodiments, the glass is comprises a laminate of two or moreglass sheets.

Compressive stresses of glasses resulting from the processes disclosedherein vary as a function of thickness of the glasses. In someembodiments, glasses having a thickness of 3 mm or less have acompressive stress of at least 80 MPa, such as at least 100 MPa, such asat least 150 MPa, such as at least 200 MPa, such as at least 250 MPa,such as at least 300 MPa, such as at least 350 MPa, such as at least 400MPa, and/or no more than 1 GPa. In contemplated embodiments, glasseshaving a thickness of 2 mm or less have a compressive stress of at least80 MPa, such as at least 100 MPa, such as at least 150 MPa, such as atleast 175 MPa, such as at least 200 MPa, such as at least 250 MPa, suchas at least 300 MPa, such as at least 350 MPa, such as at least 400 MPa,and/or no more than 1 GPa. In contemplated embodiments, glasses having athickness of 1.5 mm or less have a compressive stress of at least 80MPa, such as at least 100 MPa, such as at least 150 MPa, such as atleast 175 MPa, such as at least 200 MPa, such as at least 250 MPa, suchas at least 300 MPa, such as at least 350 MPa, and/or no more than 1GPa. In contemplated embodiments, glasses having a thickness of 1 mm orless have a compressive stress of at least of at least 80 MPa, such asat least 100 MPa, at least 150 MPa, such as at least 175 MPa, such as atleast 200 MPa, such as at least 250 MPa, such as at least 300 MPa,and/or no more than 1 GPa. In contemplated embodiments, glasses having athickness of 0.5 mm or less have a compressive stress of at least 50MPa, such as at least 80 MPa, such as at least 100 MPa, such as at least150 MPa, such as at least 175 MPa, such as at least 200 MPa, such as atleast 250 MPa, and/or no more than 1 GPa.

Glasses sheets having undergone the processes described herein may befurther processed by undergoing ion exchange to further enhance theirstrength. Ion-exchanging glasses heat strengthened as described hereinmay increase the above-described compressive stresses by at least 20MPa, such as at least 50 MPa, such as at least 70 MPa, such as at least80 MPa, such as at least 100 MPa, such as at least 150 MPa, such as atleast 200 MPa, such as at least 300 MPa, such as at least 400 MPa, suchas at least 500 MPa, such as at least 600 MPa and/or no more than 1 GPa,in some such contemplated embodiments.

In addition to thermally tempering thin glass sheets, the processes andapparatuses described herein can be used for additional processes aswell. While cooling is specifically called out, the apparatuses andprocesses could be used equally well to transfer heat into the glasssheet via a conductive method. Such a process or method is illustratedin the flow chart of FIG. 18. The method 700 there shown includes twomain steps. The first step, step 710 involves simply providing anarticle having a surface. The second step, step 720 involves heating orcooling a portion of the surface of the article up to and including theentire surface of the article. Step 720 is performed by conduction morethan by convection through a gas from or to a heat source or a heat sinksource as shown in sub-part 720 a, and is performed sufficiently tocomplete thermal conditioning of the article or the portion of thesurface of the article in sub-part 720 b, and the conduction of thecooling/heating of step 720 is performed at a high rate of heattransfer, at least 450 kW/m² of the area of the portion in sub-part 720b.

For example, an article can be thermally conditioned—i.e., either heatedor cooled—by cooling or heating a portion a portion of the surface ofthe article up to and including the entire surface of the article, theportion having an area, by conduction more than by convection, theconduction mediated through a gas to or from a heat sink or a heatsource and not through solid to solid contact, sufficiently to completea thermal conditioning of the article or of the portion of the surfaceof the article, and the conduction being performed, during at least sometime of the heating or cooling, at a rate of at least 450, 550, 650,750, 800, 900, 1000, 1100, or 1200, 1500, 2000, 3000, 4000 or even 5000or more kW per square meter.

In addition to tempering, the high rates of thermal power transfer makeit possible to for thermal processing of all kinds, including heatingand cooling during tempering, edge strengthening of glass, firing orsintering of ceramics, glasses, or other materials, and so forth.Additionally, since the heat is extracted or delivered primarily byconduction, tight control is provided over the thermal history and theheat distribution in the treated article while preserving surfacesmoothness and quality. Accordingly, it will be possible to use theapparatuses and methods of the present disclosure to intentionally varythe stress profile from the strengthening process, both in the thicknessdirection and in the directions in which the plane of the sheet lies, byvarying gaps, varying heat sink/source materials, varying heatsink/source temperatures, varying the gas mixture.

EXAMPLES

Apparatus setup—As detailed above, the apparatus comprises three zones—ahot zone, a transition zone, and a quench zone. The gaps between the topand bottom thermal bearings (heat sinks) in the hot zone and the quenchzone are set to the desired spacings. Gas flow rates in the hot zone,transition zone, and quench zone are set to ensure centering of the parton the air-bearing. The hot zone is pre-heated to the desired T₀, thetemperature from which the glass article will be subsequently quenched.To ensure uniform heating, glass articles are pre-heated in a separatepre-heating apparatus, such as a batch or continuous furnace. Generally,glass sheets are pre-heated for greater than 5 minutes prior to loadingin the hot zone. For soda lime glasses, pre-heating is done around 450°C. After the pre-heat phase, the glass article is loaded into the hotzone and allowed to equilibrate, where equilibration is where the glassis uniformly at T₀. T₀ can be determined by the tempering desired, butis generally kept in the range between the softening point and the glasstransition temperature. The time to equilibration is dependent at leaston the thickness of the glass. For example, for glass sheets ofapproximately 1.1 mm or less, equilibration occurs in approximately 10seconds. For 3 mm glass sheets, equilibration occurs in approximately 10seconds 30 seconds. For thicker sheets, up to approximately 6 mm, theequilibration time may be on the order of 60 seconds (for articlesapproximately 6 mm thick). Once the glass has equilibrated to T0, it israpidly transferred through the transition zone on air bearings and intothe quench zone. The glass article rapidly quenches in the quench zoneto a temperature below the glass transition temperature, Tg. The glasssheet can be maintained in the quench zone for any period of time from 1second, 10 seconds, or to several minutes or more, depending on thedegree of quench desired and/or the desired temperature of the glass atremoval. Upon removal the glass is optionally be allowed to cool beforehandling.

The following examples are summarized in Table V.

Example 1

A soda-lime silicate glass plate of 5.7 mm thickness is pre-heated for10 minutes at 450° C. before transferring to the hot zone where it isheld at a T₀ of 690° C. for 60 seconds. After equilibrating to T₀, it israpidly transferred to the quench zone, which has a gap of 91 μm(wherein the gap is the distance between the surface of the glass sheetand the nearest heat sink), where it is held for 10 seconds. Theresulting article has a surface compression of −312 MPa, a centraltension of 127 MPa, and a flatness of 83 μm.

Example 2

A soda-lime silicate glass plate of 5.7 mm thickness is pre-heated for10 minutes at 450° C. before transferring to the hot zone where it isheld at a T₀ of 690° C. for 60 seconds. After equilibrating it israpidly transferred to the quench zone, which has a gap of 91 lam, whereit is held for 10 seconds. The resulting article has a surfacecompression of −317 MPa, a central tension of 133 MPa, and a flatness of90 μm.

Example 3

A soda-lime silicate glass plate of 1.1 mm thickness is pre-heated for10 minutes at 450° C. before transferring to the hot zone where it isheld at a T₀ of 700° C. for 10 seconds. After equilibrating it israpidly transferred to the quench zone, which has a gap of 56 lam, whereit is held for 10 seconds. The resulting article has a surface fictivetemperature measured to be 661° C., a surface compression of −176 MPa, acentral tension of 89 MPa, a flatness of 190 μm, and a Vicker's crackingthreshold of 10-20 N.

Example 4

A soda-lime silicate glass plate of 0.55 mm thickness is pre-heated for10 minutes at 450° C. before transferring to the hot zone where it isheld at a T₀ of 720° C. for 10 seconds. After equilibrating it israpidly transferred to the quench zone, which has a gap of 25 lam, whereit is held for 10 seconds, resulting in an effective heat transfer rateof 0.184 cal/(cm²-s-° C.). The resulting article has a surfacecompression of −176 MPa, a central tension of 63 MPa, and a flatness of125 μm.

Example 5

A CORNING® GORILLA® Glass plate of 1.5 mm thickness is pre-heated for 10minutes at 550° C. before transferring to the hot zone where it is heldat a T₀ of 790° C. for 30 seconds. After equilibrating it is rapidlytransferred to the quench zone, which has a gap of 226 μm, where it isheld for 10 seconds. The glass article has an improvement in flatnessmeasured to be 113 μm pre-processing and 58 μm post-processing.

Example 6

A soda-lime silicate glass plate of 0.7 mm thickness is pre-heated for10 minutes at 450° C. before transferring to the hot zone where it isheld at a T₀ of 730° C. for 10 seconds. After equilibrating it israpidly transferred to the quench zone, which has a gap of 31 lam, whereit is held for 10 seconds, resulting in an effective heat transfer rateof 0.149 cal/(cm²-s-° C.). The resulting article has a surfacecompression of −206 MPa, a central tension of 100 MPa, and a flatness of82 μm. Upon fracture, the glass sheet is observed to “dice” (usingstandard terminology for 2 mm thickness or greater sheet dicing—i.e., a5×5 cm square of glass sheet breaks into 40 or more pieces) suggestingthat the sheet is fully tempered.

Example 7

A Borofloat-33 glass plate of 3.3 mm thickness is pre-heated for 10minutes at 550° C. before transferring to the hot zone where it is heldat a T₀ of 800° C. for 30 seconds. After equilibrating it is rapidlytransferred to the quench zone, which has a gap of 119 lam, where it isheld for 10 seconds. The resulting article has a flatness of 120 μm.Upon fracture of the part it is observed to “dice” (using standardterminology for 2 mm or greater thickness sheet dicing—i.e., a 5×5 cmsquare of glass sheet breaks into 40 or more pieces) showing that thesheet is fully tempered.

Example 8

A soda-lime silicate glass plate of 3.2 mm thickness is pre-heated for10 minutes at 450° C. before transferring to the hot zone where it isheld at a T₀ of 690° C. for 30 seconds. After equilibrating it israpidly transferred to the quench zone, which has a gap of 84 lam, whereit is held for 10 seconds. The resulting article has a surfacecompression of −218 MPa, a central tension of 105 MPa, and a flatness of84 μm.

Example 9

A soda-lime silicate glass plate of 0.3 mm thickness is pre-heated for10 minutes at 450° C. before transferring to the hot zone where it isheld at a T₀ of 630° C. for 10 seconds. After equilibrating it israpidly transferred to the quench zone, which has a gap of 159 lam,where it is held for 10 seconds. The resulting article has membranestresses which are observable by gray field polarimetry, suggesting theglass has incorporated the thermal stress.

Example 10

A CORNING® GORILLA® Glass plate of 0.1 mm thickness is pre-heated for 10minutes at 550° C. before transferring to the hot zone where it is heldat a T₀ of 820° C. for 10 seconds. After equilibrating it is rapidlytransferred to the quench zone, which has a gap of 141 μm, where it isheld for 10 seconds, resulting in an effective heat transfer rate of0.033 cal/(cm²-s-° C.). Upon fracture, the resulting article displaysbehavior consistent with a residually stressed glass.

Example 11

A soda-lime silicate glass plate of 1.1 mm thickness is pre-heated for10 minutes at 450° C. before transferring to the hot zone where it isheld at a T₀ of 700° C. for 10 seconds. After equilibrating it israpidly transferred to the quench zone, which has a gap of 65 lam, whereit is held for 10 seconds, resulting in an effective heat transfer rateof 0.07 cal/(cm²-s-° C.). The resulting article has a surface fictivetemperature measured to be 657° C., a surface compression of −201 MPa, acentral tension of 98 MPa, a flatness of 158 μm, and a Vicker's crackingthreshold of 10-20 N.

Example 12

A CORNING® GORILLA® Glass plate of 1.1 mm thickness is pre-heated for 10minutes at 550° C. before transferring to the hot zone where it is heldat a T₀ of 810° C. for 10 seconds. After equilibrating it is rapidlytransferred to the quench zone which has a gap of 86 μm, where it isheld for 10 seconds, resulting in an effective heat transfer rate of0.058 cal/(cm²-s-° C.). The resulting article has a surface fictivetemperature measured to be 711° C., a surface compression of −201 MPa, acentral tension of 67 MPa, and a Vicker's cracking threshold of 20-30 N.

Example 13

A CORNING® GORILLA® Glass plate of 1.1 mm thickness is pre-heated for 10minutes at 550° C. before transferring to the hot zone where it is heldat a T₀ of 800° C. for 10 seconds. After equilibrating it is rapidlytransferred to the quench zone, which has a gap of 91 lam, where it isheld for 10 seconds. The resulting article has a surface fictivetemperature measured to be 747° C., a surface compression of −138 MPa, acentral tension of 53 MPa, a flatness of 66 μm, and a Vicker's crackingthreshold of 20-30 N.

TABLE V Thickness Gaps CS CT Flatmaster Fictive Vickers Example (mm)Composition (um) T₀ Gas (MPa) (MPa) (um) (° C.) (N) 1 5.7 SLG 91 690Helium −312 127 83 — — 2 5.7 SLG 91 690 Helium −317 133 90 — — 3 1.1 SLG56 700 Helium −176 89 190 661.3 10-20 4 0.55 SLG 25 720 Helium −176 63125 — — 5 1.5 GG 226 790 Helium — — 113 before/ — — 58 after 6 0.7 SLG31 730 Helium −206 100 82 — — 7 3.3 Borofloat 33 119 800 Helium — — 121— — 8 3.2 SLG 84 690 Helium −218 105 81 — — 9 0.3 SLG 159 630 Helium — —— — — 10 0.1 GG 141 820 Helium — — — — — 11 1.1 SLG 65 700 Helium −20198 158 657 10-20 12 1.1 GG 86 810 Helium −201 67 — 711 20-30 13 1.1 GG91 800 Helium −138 53 66 747 20-30

Other aspects and advantages will be apparent from a review of thespecification as a whole and the appended claims.

U.S. application Ser. No. 14/814,181 filed Jul. 30, 2015 is incorporatedby reference herein in its entirety.)

What is claimed is:
 1. A thermally strengthened glass sheet: the glass sheet having a thickness, expressed in millimeters, of t, a length, expressed in millimeters, of l, and a width, expressed in millimeters, of w, t being less than l and less than w; the glass sheet having a first major surface and a second major surface separated by the thickness t, the first major surface of the sheet being flat to 100 μm total indicator run-out (TIR) along any 50 mm or less profile of the first major surface the glass sheet comprising a glass having a softening temperature, expressed in units of ° C., of T_(soft) and an annealing temperature, expressed in units of ° C., of T_(anneal), and a surface fictive temperature measured on the first major surface of the glass sheet represented by Tfs, when expressed in units of ° C.; the glass sheet having a non-dimensional surface fictive temperature parameter θs given by (Tfs−T_(anneal))/(T_(soft)−T_(anneal)), wherein the parameter θs is in the range of from 0.50 to 0.9.
 2. The glass sheet according to claim 1 wherein the parameter θs is in the range of from 0.51 to 0.9.
 3. The glass sheet according to claim 1 wherein the parameter θs is in the range of from 0.52 to 0.9.
 4. The glass sheet according to claim 1 wherein the parameter θs is in the range of from 0.60 to 0.9.
 5. The glass sheet according to claim 1, l and w each being at least 10 mm.
 6. The glass sheet according to claim 5, wherein the ratio l/t and the ratio w/t each are equal to 10/1 or more.
 7. The glass sheet according to claim 1 wherein the first major surface of the sheet is flat to 50 μm total indicator run-out (TIR) along any 50 mm or less profile of the first major surface.
 8. The glass sheet according to claim 6 wherein the first major surface has a roughness in the range of from 0.2 to 1.5 nm Ra over an area of 10×10μ.
 9. The glass sheet according to claim 1 wherein the first major surface has a coating.
 10. The glass sheet according to claim 1 wherein t is less than 2 mm.
 11. The glass sheet according to claim 1 wherein t is 0.7 mm or less.
 12. The glass sheet according to claim 1 wherein t is 0.28 mm or less.
 13. The glass sheet according to claim 1 wherein the sheet exhibits a 50% cracking threshold at 5 Newtons or greater after a 15 second Vickers Hardness Test.
 14. The glass sheet according to claim 1, wherein the compressive stress of said first major surface is larger than 150 MPa.
 15. The glass sheet according to claim 1, wherein the central tension of the sheet is larger than 75 MPa.
 16. The glass sheet according to claim 15 wherein the surface fictive temperature measured on the first surface of the sheet is at least 50° C. above a glass transition temperature of the glass.
 17. The glass sheet according to claim 15 wherein the surface fictive temperature measured on the first surface of the sheet is at least 75° C. above a glass transition temperature of the glass.
 18. The glass sheet according to claim 1, l and w each being at least 40 mm.
 19. The glass sheet according to claim 1, wherein the ratio l/t and the ratio w/t each are equal to 20/1 or more.
 20. The glass sheet according to claim 1, wherein the ratio l/t and the ratio with each are equal to 100/1 or more.
 21. The glass sheet according to claim 1 wherein t is 1 mm or less.
 22. The glass sheet according to claim 1 wherein t is 0.5 mm or less.
 23. The glass sheet according to claim 1 wherein the sheet exhibits a 50% cracking threshold at 10 Newtons or greater after a 15 second Vickers Hardness Test.
 24. The glass sheet according to claim 1, wherein the compressive stress of said first major surface is larger than 80 MPa.
 25. The glass sheet according to claim 1, wherein the compressive stress of said first major surface is larger than 100 MPa.
 26. The glass sheet according to claim 1, wherein the central tension of the sheet is larger than 40 MPa.
 27. The glass sheet according to claim 1, wherein the central tension of the sheet is larger than 50 MPa.
 28. A thermally strengthened glass sheet: the glass sheet having a thickness, expressed in millimeters, of t, a length, expressed in millimeters, of l, and a width, expressed in millimeters, of w, t being less than l and less than w; the glass sheet having a first major surface and a second major surface separated by the thickness t, the first major surface of the sheet being flat to 100 μm total indicator run-out (TIR) along any 50 mm or less profile of the first major surface the glass sheet comprising a glass having a softening temperature, expressed in units of ° C., of T_(soft) and an annealing temperature, expressed in units of ° C., of T_(anneal), and a surface fictive temperature measured on the first major surface of the glass sheet represented by Tfs, when expressed in units of ° C.; the glass sheet having a non-dimensional surface fictive temperature parameter θs given by (Tfs−T_(anneal))/(T_(soft)−T_(anneal)), wherein the parameter θs is in the range of from 0.50 to 0.9; and wherein the glass has a low temperature linear CTE, expressed in 1/° C., of α^(S) _(CTE) a high temperature linear CTE, expressed in 1/° C., of α^(L) _(CTE), an elastic modulus, expressed in GPa, of E, and a strain temperature, expressed in units of ° C., of T_(strain); the first major surface of the glass sheet having a thermally induced surface compressive stress of less than 600 MPa and greater than ${\frac{{P_{1}(h)}*t}{\left( {{P_{2}(h)} + t} \right)} \cdot E \cdot \left\lbrack {{T_{strain} \cdot \alpha_{CTE}^{s}} + {\alpha_{CTE}^{L} \cdot \left( {T_{soft} - T_{strain}} \right)}} \right\rbrack};$ in units of MPa; wherein P₁ is given by ${910.2 - {259.2 \cdot {\exp\left( {- \frac{h}{0.143}} \right)}}};$ P₂ is given by ${2.53 + \frac{23.65}{\left( {1 + \left( \frac{h}{0.00738} \right)^{1.58}} \right)}};$ and h is greater than or equal to 0.020 cal/s·cm²·° C.
 29. The glass sheet according to claim 28 wherein h is greater than or equal to 0.026 cal/s·cm²·° C.
 30. A thermally strengthened glass sheet: the glass sheet having a thickness, expressed in millimeters, of t, a length, expressed in millimeters, of l, and a width, expressed in millimeters, of w, t being less than l and less than w; the glass sheet having a first major surface and a second major surface separated by the thickness t, the first major surface of the sheet being flat to 100 total indicator run-out (TIR) along any 50 mm or less profile of the first major surface the glass sheet comprising a glass having a softening temperature, expressed in units of ° C., of T_(soft) and an annealing temperature, expressed in units of ° C., of T_(anneal), and a surface fictive temperature measured on the first major surface of the glass sheet represented by Tfs, when expressed in units of ° C.; the glass sheet having a non-dimensional surface fictive temperature parameter θs given by (Tfs−T_(anneal))/(T_(soft)−T_(anneal)), wherein the parameter Os is in the range of from 0.50 to 0.9; and wherein the glass has a low temperature linear CTE, expressed in 1/° C., of α^(S) _(CTE), a high temperature linear CTE, expressed in 1/° C., of α^(L) _(CTE), an elastic modulus, expressed in GPa, of E, and a strain temperature, expressed in units of ° C., of T_(strain); the glass sheet having a thermally induced central tension of less than 300 MPa and greater than ${\frac{{P_{1{CT}}\left( h_{CT} \right)}*t}{2\left( {{P_{2{CT}}\left( h_{CT} \right)} + t} \right)} \cdot E \cdot \left\lbrack {{T_{strain} \cdot \alpha_{CTE}^{s}} + {\alpha_{CTE}^{L} \cdot \left( {T_{soft} - T_{strain}} \right)}} \right\rbrack};$ in units of MPa; wherein P_(1CT) is given by ${910.2 - {259.2 \cdot {\exp\left( {- \frac{h_{CT}}{0.143}} \right)}}};$ P_(2CT) is given by ${2.53 + \frac{23.65}{\left( {1 + \left( \frac{h_{CT}}{0.00738} \right)^{1.58}} \right)}};$ and h_(CT) is greater than or equal to 0.020 cal/s·cm²·° C.
 31. The glass sheet according to claim 30 wherein h_(CT) is greater than or equal to 0.026 cal/s·cm²·° C. 